A bug regarding particle effects that emit from painting corners.
What I expected to happen was:
The painting would hang on the block.
What happened was:
Particles emitted from the top right and bottom left corners.
Steps to reproduce:
Place down a block.
Place a painting on the block.
Watch particles being emitted from the top right and bottom left corner.
I was experimenting with particle effects some hours ago, which includes this particle, but I switched off any contraption that might have caused this.
The particle effect is known as 'depthsuspend'.
I have had this issue as well. Facing west, I see particles (they look like "townaura") in the upper left and lower right corners of any painting.