So I was testing doubtful moral stuff in debug mode and came up with this.
They are definitely not suffocating, which is the behaviour I actually expected from Moon and Gossman, but obviously not from Dan and Vernon.
I'm going to try again in a regular save as soon as I have some spare time and update the post
Steps to Reproduce
Entomb Dupes' heads in a tile with oxygen freely flowing around them. They will just hang around.
This issue has not been confirmed by a developer yet.