Comment 15 for bug 1927868

OK, I did find evidence that this issue did begin to happen as we upgraded our Train packages just before we upgraded to Ussuri (I believe its best practice to update all packages, THEN upgrade to a new version of OpenStack)

Upgraded Neutron 15.3.0 to 15.3.3 at 16:40 EDT
2021-06-10 16:40:40 upgrade neutron-l3-agent:all 2:15.3.0-0ubuntu1~cloud2 2:15.3.3-0ubuntu1~cloud0

Evidence of gateway problems at 17:06 EDT (Might have seen this in the logs several minutes earlier but I had to purge a lot of logs from that day because of the sheer volume of them)
2021-06-10 17:06:23.440 33440 ERROR neutron.agent.l3.ha_router [-] Gateway interface for router 04a5a2c3-739f-4cf6-94bb-dd314c2a8e66 was not set up; router will not work properly

Upgraded from latest Train to latest Ussuri happened at 17:12
2021-06-10 17:12:31 upgrade neutron-l3-agent:all 2:15.3.3-0ubuntu1~cloud0 2:16.3.1-0ubuntu1~cloud0