Blocks effected by gravity, such as sand, gravel and concrete powder, get destroyed when a sticky piston pulls the block underneath them down. For example: In a triple piston extender the pistons typically have to pull the 3rd piston down; If this door were to have powder, sand or gravel, that block, (directly above the third piston), would be broken.

This could be caused by the blocks falling too fast, but I really can't say. I just know that no doors, using sand, gravel or powder, will work while this bug exists.

For the images attatched: I just want to be clear that I'm only retracting the sticky piston by turning off the lever. I was hoping to submit a video of me showing the glitch, and thus show why it needs to be patched asap, however, I was only able to attach some screenshots. The first shows 4 powder, then I retracted, (showing the block breaking in the second image), leading to the third image where the powder is now 3 high.