When the player feet are inside a block, for example, the top half of the door or inside a regular block, the player does not visually sneak and no slowness is applied, but the player cannot get out the block because the sneaking functionality is working.

Reproduction steps

Method 1

1. Place down a door
2. Stand where the door would be when you open it
3. Jump, right click the door and start holding sneak in quick succession. You should now be inside the door
4. Whilst still sneaking, try and move around.
5. Notice the player model is not sneaking and you have no slowness

Method 2

1. Create a two-block pillar
2. Stand on top of the pillar
3. Enable toggle-sneak and toggle sneak on
4. Run /tp @s ~ ~-1 ~