When two sticky pistons are stacked on top of each other and the top piston pulls a slime block while the bottom pulls any other block at the same time, there is a chance that the slime block does not retract. I managed to find a very similar report: MCPE-21826, but this one has been resolved. Apologies if I referenced the similar report incorrectly. You should be able.to recreate this bug with the setup shown in the screenshots.