When a painting is created via an item with specific Variant data (i.e; the special specific-painting items found in the Creative menu), it does not maintain its custom Variant data when broken; instead, all paintings drop a generic Painting item regardless of how they were created.

This becomes an issue when trying to leverage custom paintings in data packs; for example, if I wanted to add a custom painting variant as loot exclusive to dungeon chests, it would be rendered completely pointless since the item will revert to a generic painting as soon as the player places & breaks it again.

Either of these two solutions would help solve this issue:
A: Make paintings maintain any custom variant data when placed & broken again.
or B: Add variant-dependent loot tables to paintings. This would probably be the more complicated method, but would make paintings far more consistent with other placeable entities like armor stands.