CB used the dirty flag to construct the chunk unload event,
but then sets mustNotSave to the inverted value of the event
after calling the event without considering that the chunk may
actually be brought up to loaded status again later. Then, CB
overrides the isUnsaved method of LevelChunk to additionally
use mustNotSave.
Thus, if the chunk is not marked dirty when unloading, the
mustNotSave value will be set to true. Then, once the chunk
is reloaded and edited the dirty flag will be set. However,
when unloading the chunk finally, the isUnsaved method
will return false due to mustNotSave being true. Thus, the
chunk will never be saved.
To fix these issues, no longer make mustNotSave override
isUnsaved and always set the save flag for the chunk unload
event.
This issue started popping up recently due to the recent
change to mark chunks as not dirty after saving them, which
increased the chance of the save issue occurring in the first
place.
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