Reloading a chunk with a map inside item frame duplicates map marker

After applying my patch to MC-3416 that draws the map markers in front of the map instead of behind it, Fang Zhang was able to take this screenshot that reveals that map markers have been duplicated:

After a simple test I have a found a way to replicate this bug in vanilla:

1. Create a new regular superflat world
2. /tp 0 5 -200
3. /setworldspawn
   • (to make sure 0,0 gets unloaded)
4. /tp 5 5 5
5. Create a map and put a copy in a frame, leaving one copy in your hand.
6. /tp 5 5 200
7. /tp 5 5 5
   • Repeat last two steps twice.
8. Drop the map from your inventory and pick it up again

Your marker has now disappeared, because MC-3416 hides all markers except the first four and reloading the chunk three times has now duplicated the marker in the frame to fill slots 1-4.

I took some additional screenshots after a command block machine had teleported me back and forth for some time, which demonstrate both MC-3416 and this bug: The map marker has been duplicated and the markers are being offset in the wrong direction.

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Code analysis by Marcono1234 in this comment.