Your /etc/NetworkManager/NetworkManager.conf contains
[ifupdown]
managed=false
In that case, as I understand it, ifupdown will be left in control of interfaces defined in /e/n/i. NM interprets the "auto eth0" as meaning that eth0 should be under ifupdown control, but that line alone is not enough to make ifup do anything with eth0. So eth0 is not brought up.
Your /etc/NetworkMan ager/NetworkMan ager.conf contains
[ifupdown]
managed=false
In that case, as I understand it, ifupdown will be left in control of interfaces defined in /e/n/i. NM interprets the "auto eth0" as meaning that eth0 should be under ifupdown control, but that line alone is not enough to make ifup do anything with eth0. So eth0 is not brought up.