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