What is happening

•  When a map is loaded in a player inventory or item frame, the Decorations from the NBT of the map is presumably applied to the map ID instead of rendering specific to the map item.

• When a map with decorations is destroyed or has its decoration removed, the decorations linger on every other map of the same ID until the world is reloaded.
• When two maps with similarly named decorations and the same map ID are loaded, only the first will have its decorations render, and it propagates from maps as they're loaded, creating unexpected changes to maps for other players

What is expected:

• The decorations from the NBT would remain specific to the item in hand.
• Decorations removed from a map's NBT would remove the decoration without needing to reload the world.

Steps to reproduce:
1. Create a new world

2. Create a filled map with no map decorations at some location ((0,0} for the provided command)

3. Using a command block, generate a map with the same ID and some decorations:

/give @p minecraft:filled_map[minecraft:map_decorations={+1:{rotation: 180.0f, x: -10.0d, z: -20.0d, type: "minecraft:mansion"},+2: {rotation: 180.0f, x: -20.0d, z: 10.0d, type: "minecraft:mansion"},+3: {rotation: 180.0f, x: 20.0d, z: -10.0d, type: "minecraft:mansion"}},minecraft:map_id=0]

4. Observe the original undecorated map; it will now have all the decorations from the above map.

5. To reproduce conflicting decorations, generate a new copy of the map but with different decorations under the same decoration IDs. In this case, this map should show monuments when not conflicted.

/give @p minecraft:filled_map[minecraft:map_decorations={+1:{rotation: 180.0f, x: -20.0d, z: -40.0d, type: "minecraft:monument"},+2: {rotation: 180.0f, x: -40.0d, z: 20.0d, type: "minecraft:monument"},+3: {rotation: 180.0f, x: 40.0d, z: -20.0d, type: "minecraft:monument"}},minecraft:map_id=0]

6. To reproduce maps inheriting decorations from multiple maps, simply generate multiple copies of the map with different decorations and decoration IDs. This command will add 3 red x's to all the other maps because the decoration IDs don't conflict.

/give @p minecraft:filled_map[minecraft:map_decorations={+4:{rotation: 180.0f, x: -30.0d, z: -30.0d, type: "minecraft:red_x"},+5: {rotation: 180.0f, x: -30.0d, z: 30.0d, type: "minecraft:red_x"},+6: {rotation: 180.0f, x: 30.0d, z: -30.0d, type: "minecraft:red_x"}},minecraft:map_id=0]