Good morning.

Pressing the crouch-button on any chest type with any trapdoor type over it, standing in the trapdoor and trying to move off the chest in any direction will make you fall off. This has been tested on multiplayer servers too.

Trying to crouch off a chest without a trapdoor over it makes it act like a block and wont make you fall off.

My theory is that the trapdoor is trying to push the player off the chest and because the chest has a smaller hitbox, it succeeds.

This bug can be re-created for testing by the images attached with this post.