Flying south by southwest into the world into chunks that have not been visited since some combination of 1.14 (original generating version) to 1.17 causes the server to eventually permanently stop responding; logged-in clients will be disconnected but will appear to still be logged into the server until it is restarted/killed. Stack dump appears to show a near-if-not-infinite recursion occurring on the main server thread(s) involving FluidTypeFlowing, BlockFluid, and a craftbukkit function CraftBlock.setTypeAndData. Only plugin installed is AutoWorldBackup; only datapacks are well-known and previously functioning VanillaTweaks and similar crafting recipe packs. No previous problems were had running with this configuration.

World was force-upgraded to 1.18 earlier so all region files have already been converted to 1.18 format.