Saving a hotbar with a filled map in one world and restoring it in another world causes undefined behaviour of the map

Saved hotbars storage contains NBT's of maps and other items.

This bug is caused by the system of map data storage in dedicated files "<world_name>/data/map_#.dat".

This may happen (very probably) on older versions but I did not check it.

Steps to reproduce the bug:

1. Create world "World A" and do the following:

2. Run /give @s minecraft:map

3. Right-click the map to get a filled map

4. Check how the map renders - it should display terrain (attachment WorldA.png)

5. Save hotbar (press C+1)

6. Exit "World A"

7. Create "World B" and open it, do the following:

8. Load the previously saved hotbar (press X+1)

9. Filled map should appear in the hotbar, check how it renders now - it's empty (and that is the bug) (attachment WorldB.png)

Potential fixes:

• Store all the map data inside an NBT of a single item

• Implement a way of transferring maps between worlds.

• Ignore this bug (mark as "Won't fix")

• Not allow to put filled maps in saved hotbars