Flying around alone for a while with the server using Java 11 again, I was not able to reproduce the crash. I also couldn't find anything specifically relevant in the Mojang tracker to this issue, although there have been several cases in the past of "java.util.ConcurrentModificationException" being raised and crashing the whole game that were fixed in previous releases. As a very crutchy band-aid, I have modified our service script to restart the game server if it detects an unexpected shutdown.
For turtle, this means I tweaked /etc/systemd/system/multi-user.target.wants/minecraft.service to add:
I tested these settings by forcing the game to halt using the "/stop" op command, and within a minute it was right back up again.
It's something. ¯\_(ツ)_/¯
I'll keep an eye on the "crash-reports" folder over the next couple of days, and I'll file a bug for Mojang with anything new that comes up henceforth.