Note that this is for 1.16.100 but this option is not available in the drop down box above!

The game will freeze - either for ages (1+ minute) and recover, or require a restart.   Players can still move around but the world is frozen in time.  Mobs don't move and there is no sound when placing/breaking blocks.   Most of the time this happens when someone leaves the server, however sometimes it is random.   Also it can't necessarily be re-created, eg a player left and the server crashed - I asked the same player to log back on and off again, but the second time the server did NOT crash.   Their inventory and physical location in the world did not change.   All addons have been removed but the problem persists. 

It's very similar to the symptoms the game displays when it cannot write text to the console (like if you've half typed a command and forget to press enter).   However in this case the console remains responsive, I can still run command and get responses back even though the world has frozen.