See video for and the zipped world download in the attachment for greater understanding:

https://youtu.be/hTHWUE328cI

1. Build a 2x2x3 hole and put about 20 or more mobs in it
2. Use Sticky Pistons to crush the mobs in it
3. Make the Pistons retract before the mobs die
4. A small amount of mobs get pushed alongside the blocks the piston is pulling back, making them suffocate in said block instead of being unaffected like the rest.

Also tested with Zombies, same outcome.

Apart from using Optifine 1.20.4 Version, it is purely vanilla unmodded minecraft. Additionally, I tested it in vanilla minecraft as well, the same thing happens. Some can glitch through the wall, and others suffocate in the wall (in the video both happens with 1 blaze each).