Sticky piston pulls mob into itself

When a sticky piston pushes a block onto a mob to suffocate it, the expected result would be that when the sticky piston extends the mob suffocates and when it retracts the mob no longer suffocates. Instead though when it retracts it glitches and the mob (a spider in my case) goes back with it into the block that the sticky piston had pushed on the mob and then suffocates.