In RegionFileCache there is a missing close() call to a region file that is evicted from the cache.

See here: (note these are spigot mappings, without changes from spigot/craftbukkit)

https://gist.github.com/Spottedleaf/dec40400015df81ad2081df35905e3a8

The fix is to simply call the close() method on the evicted RegionFile.