Paintings are broken. That's it. Hitboxes are wrongly placed (or the painting is wrongly displaced when rendered), they clip through blocks, and due the hitbox being wrong, it's hard to break them, and sometimes Minecraft stops rendering them, as it "thinks" is outside the FOV. Pictures attached.
Also, the first time loading a world with paintings created on a previous version (16w21b, actually), the server output showed this:

[18:32:07] [Server thread/WARN]: Wrong location! (9, 15) should be (8, 15), yc['Painting'/241, l='1.9 Snapshot World', x=144,00, y=65,00, z=250,00]
[18:32:07] [Server thread/WARN]: Tried to add entity Painting but it was marked as removed already

and the paintings were broken (I had two. Found one on the floor as a drop, the other one just disappeared)

EDIT:
Added screenshots of paintings placed facing west and east.
EDIT 2:
Added screenshot of painting clipping through blocks.