Several servers stuck on a reboot cycle after Windows Update last night (Tuesday - Wednesday).
This appears to be a combination of 2019-12 Security Monthly Quality Rollup for Windows Server 2012 for x64-based Systems (KB4530691)
and
2019-12 Servicing Stack Update for Windows Server 2012 for x64-based Systems (KB4532920)
The solution is to boot the server in safe mode and allow the update to fail. This can take a long time.
I think that the way to avoid this is to manually install 2019-12 Servicing Stack Update for Windows Server 2012 for x64-based Systems (KB4532920) before the rollup, but I am still testing this.
I don’t imagine this will impact too many users, but hopefully the above helps someone.