The Bug

Additional/custom painting variants in data pack are ignored when uploading a data pack to a Realm in Java Edition 1.21. This causes the painting variant to no longer appear in the Creative inventory, reverts placed custom painting variants to "Albanian" variant, and prevents placing of paintings entirely.

How to Reproduce

1. Download a data pack that adds additional painting variant(s), for example the attached data pack
2. Add this data pack to your Realm in 1.21
3. In-game, look for the added painting variant(s) in the Creative inventory or attempt to place a painting on any sufficiently sized wall
4. Notice that the custom painting variant doesn't appear in the Creative inventory and all paintings cannot be placed on any size of surface
5. Now try to add the same data pack to a singleplayer world in 1.21
6. Notice that the painting variant(s) appears in the Creative inventory and paintings can be placed on walls as intended