Moving non full blocks pushes the player out

Steps to Reproduce:

1. Place a piston/sticky piston facing down, 4 blocks off the ground;
2. Place a top slab underneath it,
3. Stand under the slab;
4. Activate the piston.

Observed Results:
When the piston extends, the player is briefly pushed out of the space and then is forced to crouch. If the player was standing too off-center, they will be completely pushed out before being forced to crouch.

Expected Results:
When the piston extends, the player is immediately forced to crouch due to being in a space less than 1.8 blocks tall and is not pushed to the sides at all.

Screenshots/Videos attached: Yes

Notes: