After applying my patch to MC-3416 that draws the map markers in front of the map instead of behind it, bbbbbbbbba 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.

Code analysis by Marcono1234 in this comment.