I have a shulker box full of filled, level 0 maps, and seemingly at random some of the maps become "unknown map" (all of the filled in data and the location (position in the world) of the map remain unchanged though). And this doesn't affect all maps, only some are changed. Everything is set back to normal after taking the unknown map out of the shulker box and placing it in the inventory.
EDIT: Seems to affect all container blocks as well, when placed. If the map is in the player's inventory or in an item frame, then the issue isn't present. The maps becoming unknown map only happens when relogging as well. Even ender chests, which have storage exclusive to each player, are affected.