Not sure how to explain so I'll list how it happens.

How to replicate bug:

-Place 2 sticky pistons facing sideways on top of one another

-Place a slime block on the top piston, and any other block on the bottom

-Power the top piston from behind USING A REPEATER* (as shown in image)

-Unpower the top piston and the other block retracts, but the slime block does not

*Also happens with comparator, redstone going into a block behind top piston, or a redstone block placed behind top piston