Brian, 

The DHCP agent is configured as follows:

# grep -Ev "^#|^$" /etc/neutron/dhcp_agent.ini
[DEFAULT]
interface_driver = linuxbridge
dhcp_driver = neutron.agent.linux.dhcp.Dnsmasq
enable_isolated_metadata = true
[agent]
availability_zone = openstack-network
[ovs]

I.e. enable_isolated_metadata is enabled (recommended setting: https://docs.openstack.org/neutron/wallaby/install/controller-install-option1-ubuntu.html), force_metadata is disabled (default setting). If we disable isolated metadata, what is going to break? 

A low-MTU network has just an IPv4 address and no IPv6 link-local addresses, IPv6 is disabled:

# ip netns exec qdhcp-c9b96063-3f8f-41b2-9783-ac73d37894b0 ip a li
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host
       valid_lft forever preferred_lft forever
2: ns-bbd05a6d-0b@if675: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1279 qdisc noqueue state UP group default qlen 1000
    link/ether fa:16:3e:3d:ee:c4 brd ff:ff:ff:ff:ff:ff link-netnsid 0
    inet 10.10.10.4/24 brd 10.10.10.255 scope global ns-bbd05a6d-0b
       valid_lft forever preferred_lft forever

# ls /proc/sys/net/ipv6/conf/vxlan-3947/
ls: cannot access '/proc/sys/net/ipv6/conf/vxlan-3947/': No such file or directory

"Healthy" networks have IPv6 link-local addresses and IPv6 is enabled:

# ip netns exec qdhcp-8abc13db-565b-4640-9507-819d6ef520ef ip a li
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host
       valid_lft forever preferred_lft forever
2: ns-fc876933-90@if90: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
    link/ether fa:16:3e:36:d5:40 brd ff:ff:ff:ff:ff:ff link-netnsid 0
    inet 10.10.10.2/24 brd 10.10.10.255 scope global ns-fc876933-90
       valid_lft forever preferred_lft forever
    inet 169.254.169.254/32 brd 169.254.169.254 scope global ns-fc876933-90
       valid_lft forever preferred_lft forever
    inet6 fe80::a9fe:a9fe/64 scope link
       valid_lft forever preferred_lft forever
    inet6 fe80::f816:3eff:fe36:d540/64 scope link
       valid_lft forever preferred_lft forever

# ls /proc/sys/net/ipv6/conf/vxlan-4952/
accept_dad                  accept_ra_rtr_pref            drop_unsolicited_na          max_desync_factor                  router_probe_interval             temp_prefered_lft
accept_ra                   accept_redirects              enhanced_dad                 mc_forwarding                      router_solicitation_delay         temp_valid_lft
accept_ra_defrtr            accept_source_route           force_mld_version            mldv1_unsolicited_report_interval  router_solicitation_interval      use_oif_addrs_only
accept_ra_from_local        addr_gen_mode                 force_tllao                  mldv2_unsolicited_report_interval  router_solicitation_max_interval  use_tempaddr
accept_ra_min_hop_limit     autoconf                      forwarding                   mtu                                router_solicitations
accept_ra_mtu               dad_transmits                 hop_limit                    ndisc_notify                       seg6_enabled
accept_ra_pinfo             disable_ipv6                  ignore_routes_with_linkdown  ndisc_tclass                       seg6_require_hmac
accept_ra_rt_info_max_plen  disable_policy                keep_addr_on_down            proxy_ndp                          stable_secret
accept_ra_rt_info_min_plen  drop_unicast_in_l2_multicast  max_addresses                regen_max_retry                    suppress_frag_ndisc

Pastebin of the above: https://pastebin.com/ihXedc0U