To replicate:

1. Place two blocks with a one block space in between.
2. On both sides of the gap, place a painting so the two paintings are facing each other.
3. Disconnect from the world, (or travel far enough that the chunk unloads, or enter the End or the Nether) and come back.
4. The first painting you placed will plop as an item. Only the last painting you placed will remain.

Which is the bug? The ability to place two paintings in the same block space? The fact the first painting does not plop immediately? Or should the paintings not be plopping at all?

I tested with a friend, and as long as one person remains logged in and the chunk is loaded, the paintings are both stable. But if both people leave, and the chunk is unloaded, when we return the first painting placed will plop. We used a variety of blocks, such as stairs and polished andesite etc. The results were the same in every case. If two paintings occupy the same block space, the first one placed will plop when the chunk loads again from scratch. Using F3+A to reload the chunk manually has no effect on the paintings, since the chunk is never fully unloaded.