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