Player can still move at normal speed and sprint while sneaking in a 1.5 blocks tall space

The bug

When the player is standing in a 1.5 blocks tall space and releases the sneak key (usually Shift), it can move at normal speed and even sprint.

To reproduce

1. Sneak below a half slab
2. Release the sneak key
3. Start walking or sprinting
4. You move at normal walking / sprinting speed