A simple solution to problems on already created maps is to remove all green dots, and make the players update the maps in item frames after this fix.
The green dot should disappear when the player picks up the map from an item frame. Otherwise, it will fill the map with green dots with locations that don't have maps in item frames. It should work with locked or unlocked maps, because that green dot appears on both types when the map is placed in an item frame.
If it has been exposed to sunlight, the helmet breaks after a while.
Maybe the sunlight was the cause.
I searched a lot but didn't find anything similar.
But this is a duplicate.
I translate the issue to English now.
This shouldn't be a bug, it's like the flying candle.