Player markers disappear from all viewed copies of a map ID when at least one of the copies has been renamed.

To recreate this bug:

1. Create a new world (survival mode, cheats on)
2. Create a new map (spawn it, use it)
3. Duplicate the map (spawn a cartography table, a stack of maps, and duplicate it a few times)
4. Share a copy with a friend (confirm that it works properly)
5. Rename the other copies (spawn an anvil, rename the maps)
6. Player markers disappear!
7. Have the friend discard the unnamed copy
8. Map resumes working as expected for the remaining player!

More irregular behavior happens when you continue playing with this:

1. Share one of the named copies
2. The friend is not shown on the map. When they explore, terrain is uncovered in a disjointed fashion (See attached image, "After sharing...")