The bug
If you are inside a block (ex. closing a door or fence gate on yourself, letting a sand block fall on you), you cannot sneak or unsneak (if you are already sneaking).
Linked issues
is duplicated by 18
relates to 6
MC-149259
Crouching inside fence gate, but no crouching hit box or visual
Resolved
MC-165423
Allows you to walk at the same speed while being crouched
Open
MC-131942
Player's hitbox doesn't change while crouching inside of some blocks
Resolved
MC-157183
Player sneaking animation is not showing when clipping through a block
Open
MC-195736
Sneaking while a transparent block is placed at feet causes player to appear not sneaking but hitbox remains shorter
Open
1 more links
Fixing this causes MC-147681, which is worse, so we leave this one for now.