So far this has happened to me two times. When I'm in the nether and make a new nether portal, I start suffocating in the portal (I don't know for sure if it is the portal, as I die before I can even check) I also don't know specifically whats going on in the code when doing this, but it has happened to me the past times ive went into a new nether portal (I think if you built a new portal before 1.21 its ok, and if you made a portal in the overworld, that also wont cause the suffocation, so this bug was only introduced in 1.21)