So from the logs it looks like network manager is losing the connection. On boot glean creates the network-scripts/ifcfg-ens3 config. When NetworkManager starts up, it creates a connection called 'System ens3'

Jul  1 13:49:29 centos-8-inap-mtl01-0017550599 systemd[1]: Started Network Manager Script Dispatcher Service.
Jul  1 13:49:29 centos-8-inap-mtl01-0017550599 NetworkManager[1023]: <info>  [1593611369.2798] device (lo): carrier: link connected
Jul  1 13:49:29 centos-8-inap-mtl01-0017550599 NetworkManager[1023]: <info>  [1593611369.2800] manager: (lo): new Generic device (/org/freedesktop/NetworkManager/Devices/1)
Jul  1 13:49:29 centos-8-inap-mtl01-0017550599 NetworkManager[1023]: <info>  [1593611369.2812] manager: (ens3): new Ethernet device (/org/freedesktop/NetworkManager/Devices/2)
Jul  1 13:49:29 centos-8-inap-mtl01-0017550599 NetworkManager[1023]: <info>  [1593611369.2822] device (ens3): state change: unmanaged -> unavailable (reason 'managed', sys-iface-state: 'external')
Jul  1 13:49:29 centos-8-inap-mtl01-0017550599 kernel: IPv6: ADDRCONF(NETDEV_UP): ens3: link is not ready
Jul  1 13:49:29 centos-8-inap-mtl01-0017550599 NetworkManager[1023]: <info>  [1593611369.2873] device (ens3): carrier: link connected
Jul  1 13:49:29 centos-8-inap-mtl01-0017550599 NetworkManager[1023]: <info>  [1593611369.2934] device (ens3): state change: unavailable -> disconnected (reason 'none', sys-iface-state: 'managed')
Jul  1 13:49:29 centos-8-inap-mtl01-0017550599 NetworkManager[1023]: <info>  [1593611369.2970] policy: auto-activating connection 'System ens3' (21d47e65-8523-1a06-af22-6f121086f085)
Jul  1 13:49:29 centos-8-inap-mtl01-0017550599 NetworkManager[1023]: <info>  [1593611369.2978] device (ens3): Activation: starting connection 'System ens3' (21d47e65-8523-1a06-af22-6f121086f085)
Jul  1 13:49:29 centos-8-inap-mtl01-0017550599 NetworkManager[1023]: <info>  [1593611369.2979] device (ens3): state change: disconnected -> prepare (reason 'none', sys-iface-state: 'managed')
Jul  1 13:49:29 centos-8-inap-mtl01-0017550599 NetworkManager[1023]: <info>  [1593611369.2983] manager: NetworkManager state is now CONNECTING
Jul  1 13:49:29 centos-8-inap-mtl01-0017550599 NetworkManager[1023]: <info>  [1593611369.2985] device (ens3): state change: prepare -> config (reason 'none', sys-iface-state: 'managed')
Jul  1 13:49:29 centos-8-inap-mtl01-0017550599 NetworkManager[1023]: <info>  [1593611369.2990] device (ens3): state change: config -> ip-config (reason 'none', sys-iface-state: 'managed')
Jul  1 13:49:29 centos-8-inap-mtl01-0017550599 NetworkManager[1023]: <info>  [1593611369.3043] device (ens3): state change: ip-config -> ip-check (reason 'none', sys-iface-state: 'managed')
Jul  1 13:49:29 centos-8-inap-mtl01-0017550599 NetworkManager[1023]: <info>  [1593611369.3083] device (ens3): state change: ip-check -> secondaries (reason 'none', sys-iface-state: 'managed')
Jul  1 13:49:29 centos-8-inap-mtl01-0017550599 NetworkManager[1023]: <info>  [1593611369.3086] device (ens3): state change: secondaries -> activated (reason 'none', sys-iface-state: 'managed')
Jul  1 13:49:29 centos-8-inap-mtl01-0017550599 NetworkManager[1023]: <info>  [1593611369.3091] manager: NetworkManager state is now CONNECTED_LOCAL
Jul  1 13:49:29 centos-8-inap-mtl01-0017550599 NetworkManager[1023]: <info>  [1593611369.3101] manager: NetworkManager state is now CONNECTED_SITE
Jul  1 13:49:29 centos-8-inap-mtl01-0017550599 NetworkManager[1023]: <info>  [1593611369.3103] policy: set 'System ens3' (ens3) as default for IPv4 routing and DNS
Jul  1 13:49:29 centos-8-inap-mtl01-0017550599 NetworkManager[1023]: <info>  [1593611369.3105] device (ens3): Activation: successful, device activated.
Jul  1 13:49:29 centos-8-inap-mtl01-0017550599 NetworkManager[1023]: <info>  [1593611369.3112] manager: NetworkManager state is now CONNECTED_GLOBAL
Jul  1 13:49:29 centos-8-inap-mtl01-0017550599 NetworkManager[1023]: <info>  [1593611369.3122] manager: startup complete


When we restart legacy network, we see that the connection is now getting a default 'Wired connection 1'


Jul  1 14:15:25 centos-8-inap-mtl01-0017550599 systemd[1]: Starting LSB: Bring up/down networking...
Jul  1 14:15:25 centos-8-inap-mtl01-0017550599 network[48127]: WARN      : [network] You are using 'network' service provided by 'network-scripts', which are now deprecated.
Jul  1 14:15:25 centos-8-inap-mtl01-0017550599 network[48140]: You are using 'network' service provided by 'network-scripts', which are now deprecated.
Jul  1 14:15:25 centos-8-inap-mtl01-0017550599 network[48127]: WARN      : [network] 'network-scripts' will be removed in one of the next major releases of RHEL.
Jul  1 14:15:25 centos-8-inap-mtl01-0017550599 network[48141]: 'network-scripts' will be removed in one of the next major releases of RHEL.
Jul  1 14:15:25 centos-8-inap-mtl01-0017550599 network[48127]: WARN      : [network] It is advised to switch to 'NetworkManager' instead for network management.
Jul  1 14:15:25 centos-8-inap-mtl01-0017550599 network[48142]: It is advised to switch to 'NetworkManager' instead for network management.
Jul  1 14:15:25 centos-8-inap-mtl01-0017550599 NetworkManager[1023]: <info>  [1593612925.4888] device (ens3): state change: activated -> deactivating (reason 'connection-removed', sys-iface-state: 'managed')
Jul  1 14:15:25 centos-8-inap-mtl01-0017550599 NetworkManager[1023]: <info>  [1593612925.4893] manager: NetworkManager state is now DISCONNECTING
Jul  1 14:15:25 centos-8-inap-mtl01-0017550599 dbus-daemon[738]: [system] Activating via systemd: service name='org.freedesktop.nm_dispatcher' unit='dbus-org.freedesktop.nm-dispatcher.service' requested by ':1.3' (uid=0 pid=1023 comm="/usr/sbin/NetworkManager --no-daemon " label="system_u:system_r:NetworkManager_t:s0")
Jul  1 14:15:25 centos-8-inap-mtl01-0017550599 NetworkManager[1023]: <info>  [1593612925.4912] audit: op="connections-reload" pid=48177 uid=0 result="success"
Jul  1 14:15:25 centos-8-inap-mtl01-0017550599 systemd[1]: Starting Network Manager Script Dispatcher Service...
Jul  1 14:15:25 centos-8-inap-mtl01-0017550599 dbus-daemon[738]: [system] Successfully activated service 'org.freedesktop.nm_dispatcher'
Jul  1 14:15:25 centos-8-inap-mtl01-0017550599 systemd[1]: Started Network Manager Script Dispatcher Service.
Jul  1 14:15:25 centos-8-inap-mtl01-0017550599 NetworkManager[1023]: <info>  [1593612925.5158] device (ens3): state change: deactivating -> disconnected (reason 'connection-removed', sys-iface-state: 'managed')
Jul  1 14:15:25 centos-8-inap-mtl01-0017550599 NetworkManager[1023]: <info>  [1593612925.5205] settings: (ens3): created default wired connection 'Wired connection 1'
Jul  1 14:15:25 centos-8-inap-mtl01-0017550599 NetworkManager[1023]: <info>  [1593612925.5207] manager: NetworkManager state is now DISCONNECTED
Jul  1 14:15:25 centos-8-inap-mtl01-0017550599 NetworkManager[1023]: <info>  [1593612925.5211] policy: set-hostname: current hostname was changed outside NetworkManager: 'standalone.localdomain'
Jul  1 14:15:25 centos-8-inap-mtl01-0017550599 NetworkManager[1023]: <info>  [1593612925.5227] policy: auto-activating connection 'Wired connection 1' (c073f3b1-0f9c-3f80-a77b-6ccff1be72e5)
Jul  1 14:15:25 centos-8-inap-mtl01-0017550599 NetworkManager[1023]: <info>  [1593612925.5232] device (ens3): Activation: starting connection 'Wired connection 1' (c073f3b1-0f9c-3f80-a77b-6ccff1be72e5)
Jul  1 14:15:25 centos-8-inap-mtl01-0017550599 NetworkManager[1023]: <info>  [1593612925.5234] device (ens3): state change: disconnected -> prepare (reason 'none', sys-iface-state: 'managed')
Jul  1 14:15:25 centos-8-inap-mtl01-0017550599 NetworkManager[1023]: <info>  [1593612925.5238] manager: NetworkManager state is now CONNECTING
Jul  1 14:15:25 centos-8-inap-mtl01-0017550599 NetworkManager[1023]: <info>  [1593612925.5240] device (ens3): state change: prepare -> config (reason 'none', sys-iface-state: 'managed')
Jul  1 14:15:25 centos-8-inap-mtl01-0017550599 NetworkManager[1023]: <info>  [1593612925.5247] device (ens3): state change: config -> ip-config (reason 'none', sys-iface-state: 'managed')
Jul  1 14:15:25 centos-8-inap-mtl01-0017550599 NetworkManager[1023]: <info>  [1593612925.5251] dhcp4 (ens3): activation: beginning transaction (timeout in 45 seconds)


Later on this node when it's rebooted, glean runs again to generate ifcfg-ens3 again
Jul  1 15:25:31 centos-8-inap-mtl01-0017550599 glean.sh[732]: DEBUG:glean:Starting glean
Jul  1 15:25:31 centos-8-inap-mtl01-0017550599 glean.sh[732]: DEBUG:glean:Detected distro : centos
Jul  1 15:25:31 centos-8-inap-mtl01-0017550599 glean.sh[732]: DEBUG:glean:Configuring with NetworkManager
Jul  1 15:25:31 centos-8-inap-mtl01-0017550599 glean.sh[732]: DEBUG:glean:Found network_info file /mnt/config/openstack/latest/network_data.json
Jul  1 15:25:31 centos-8-inap-mtl01-0017550599 glean.sh[732]: DEBUG:glean:Probing system interfaces
Jul  1 15:25:31 centos-8-inap-mtl01-0017550599 glean.sh[732]: DEBUG:glean:Only considering interface ens3 from arguments
Jul  1 15:25:31 centos-8-inap-mtl01-0017550599 glean.sh[732]: DEBUG:glean:Interface matched: ens3 (fa:16:3e:fa:da:5e)
Jul  1 15:25:31 centos-8-inap-mtl01-0017550599 glean.sh[732]: DEBUG:glean:fa:16:3e:fa:da:5e configured via config-drive
Jul  1 15:25:31 centos-8-inap-mtl01-0017550599 glean.sh[732]: DEBUG:glean:Writing output files
Jul  1 15:25:31 centos-8-inap-mtl01-0017550599 glean.sh[732]: DEBUG:glean:Writing output file : /etc/sysconfig/network-scripts/ifcfg-ens3
Jul  1 15:25:31 centos-8-inap-mtl01-0017550599 glean.sh[732]: DEBUG:root:Restoring selinux context for /etc/sysconfig/network-scripts/ifcfg-ens3
Jul  1 15:25:31 centos-8-inap-mtl01-0017550599 glean.sh[732]: DEBUG:glean: ... done
Jul  1 15:25:31 centos-8-inap-mtl01-0017550599 glean.sh[732]: DEBUG:glean:Done!
Jul  1 15:25:31 centos-8-inap-mtl01-0017550599 systemd[1]: Started Glean for interface ens3 with NetworkManager.


and then we see that 'System ens3' is back

Jul  1 15:25:31 centos-8-inap-mtl01-0017550599 NetworkManager[890]: <info>  [1593617131.9123] device (ens3): state change: unavailable -> disconnected (reason 'none', sys-iface-state: 'managed')
Jul  1 15:25:31 centos-8-inap-mtl01-0017550599 NetworkManager[890]: <info>  [1593617131.9132] policy: auto-activating connection 'System ens3' (21d47e65-8523-1a06-af22-6f121086f085)
Jul  1 15:25:31 centos-8-inap-mtl01-0017550599 NetworkManager[890]: <info>  [1593617131.9140] device (ens3): Activation: starting connection 'System ens3' (21d47e65-8523-1a06-af22-6f121086f085)
Jul  1 15:25:31 centos-8-inap-mtl01-0017550599 NetworkManager[890]: <info>  [1593617131.9141] device (ens3): state change: disconnected -> prepare (reason 'none', sys-iface-state: 'managed')
Jul  1 15:25:31 centos-8-inap-mtl01-0017550599 NetworkManager[890]: <info>  [1593617131.9145] manager: NetworkManager state is now CONNECTING
Jul  1 15:25:31 centos-8-inap-mtl01-0017550599 NetworkManager[890]: <info>  [1593617131.9147] device (ens3): state change: prepare -> config (reason 'none', sys-iface-state: 'managed')
Jul  1 15:25:31 centos-8-inap-mtl01-0017550599 NetworkManager[890]: <info>  [1593617131.9219] device (ens3): state change: config -> ip-config (reason 'none', sys-iface-state: 'managed')
Jul  1 15:25:31 centos-8-inap-mtl01-0017550599 NetworkManager[890]: <info>  [1593617131.9243] device (ens3): state change: ip-config -> ip-check (reason 'none', sys-iface-state: 'managed')
Jul  1 15:25:31 centos-8-inap-mtl01-0017550599 NetworkManager[890]: <info>  [1593617131.9292] device (ens3): state change: ip-check -> secondaries (reason 'none', sys-iface-state: 'managed')
Jul  1 15:25:31 centos-8-inap-mtl01-0017550599 NetworkManager[890]: <info>  [1593617131.9295] device (ens3): state change: secondaries -> activated (reason 'none', sys-iface-state: 'managed')
Jul  1 15:25:31 centos-8-inap-mtl01-0017550599 NetworkManager[890]: <info>  [1593617131.9299] manager: NetworkManager state is now CONNECTED_LOCAL
Jul  1 15:25:31 centos-8-inap-mtl01-0017550599 NetworkManager[890]: <info>  [1593617131.9309] manager: NetworkManager state is now CONNECTED_SITE
Jul  1 15:25:31 centos-8-inap-mtl01-0017550599 NetworkManager[890]: <info>  [1593617131.9311] policy: set 'System ens3' (ens3) as default for IPv4 routing and DNS
Jul  1 15:25:31 centos-8-inap-mtl01-0017550599 NetworkManager[890]: <info>  [1593617131.9313] device (ens3): Activation: successful, device activated.
Jul  1 15:25:31 centos-8-inap-mtl01-0017550599 NetworkManager[890]: <info>  [1593617131.9319] manager: NetworkManager state is now CONNECTED_GLOBAL
Jul  1 15:25:31 centos-8-inap-mtl01-0017550599 NetworkManager[890]: <info>  [1593617131.9324] manager: startup complete