h3, The Bug
When standing on the step of a stair in front of a fence with a trap door open on top of it, the player gets forced to stand when it closes. But if you are holding crouch when it closes, the POV and the hitbox of the player is still crouched while the entity of the player stands.

Reproduce

• Create the setup with the trapdoor, fence and stairs in images below
• Press F3 + B
• Open the trapdoor
• Crouch next to the fence on the stairs
•  Close the trapdoor and un-crouch

Observed Result

The hitbox was still crouched despite the player not being crouched

Expected Result

The hitbox would return back to normal