The Bug

Usually sticky pistons pull blocks back that are placed in front of them. A piston, that is powered using a short pulse, however, leaves its block behind when retracting. This behavior broke in this week's snapshot.
This was an actually intended feature, see: MC-5726.

Speculations

I can imagine that this happend by accident when cleaning the code. It was the case before, that a sticky piston that updated and is no longer powered, would tell the block in front of it to immediately turn from block 36 to its normal block again. Maybe some rewriting changed when that happens, and thus the block now turns into a normal block before the piston starts retracting again.