If you use sand with an observer, you will get this:
https://youtu.be/RLc5d_PywvE
Comments 5
Looks like intended behavior to me. The piston merely pushed one of the sand entities so it fell further away and destroyed the others which got pushed into the already-now-blocks-again sand. I think this issue can be closed.
I'd like to know what the bug is, there appeared to be nothing buggy in that video