FixesPaperMC/Paper#883 same issue as MinecraftForge/MinecraftForge#4386
A more detailed anaylsis of what is probably going on, courtesy of
@bs2609 and the MCForge Issue Tracker is:
When a chunk is unloaded, the entities and tile entities it contains are
marked for removal. The actual removal (from the world) occurs later,
when the world ticks its entities.
Conversely, when a chunk is loaded, it generally adds its entities to
the world promptly, without queuing.
Here's the normal sequence of events:
Chunk unloaded
Old entities removed
Chunk loaded
New entities added
However, what can happen:
Chunk unloaded
Chunk loaded
New entities added
Old entities removed
This occurs when an unloaded chunk is reloaded before its corresponding
entities have been removed.
Shane Freeder
Zach Brown