If you crouch, your hitbox now shrinks to 1.5 blocks tall. If you then crawl under a 1.5 block gap, then even when you release the crouch button, your hitbox remains small until you crawl where there's more room. But your third-person model stands up immediately, with its head in the slab above, even though your hitbox doesn't reflect that. The model should remain crouched as long as the hitbox is shrunk.