Wardens can spawn in narrow places, which causes them to suffocate. You can see this when activating a shrieker around certain area with less than 3 blocks high of air.

Expected result
Wardens wouldn't be able to spawn in places with lower height than their collision box.

How to reproduce

1. Fill a certain area with stone and only let a layer of air, see the attached picture.
2. Place a shrieker around and activate it to summon the warden.
3. See how the warden is able to spawn in those narrow spots.