The bug

Banner markers can be added to and removed from maps created in a different dimension. This should probably not be possible, since it creates the following bugs:

• Markers added in a different dimension are gone when switching the dimension
  1. Create map in overworld
  2. Switch to the nether
  3. Add banner marker
  4. Switch back to overworld
• Can remove markers
  1. Create map in overworld
  2. Add banner marker at point P
  3. Switch to the nether
  4. Click banner at point P in the nether