Playing on a Realm with a friend on 1.21.2, I warped to the nether at a new location which has caused me to suffocate in an alleged wall before I could interact with the game. After hearing the damage sound in the loading screen, I was shown a death animation in a floating nether portal and immediately warped back to the last bed I slept in. There was no wall in sight.

I've recorded a clip of this incident: https://youtu.be/D4rspZYPgFY

I would guess I was spawned in the nether before the world has finished generating at a new location. I have entered the nether before when the world was a local one without issue, this was the first time I warped to the nether on the Realm at a new location.

The fix would probably be to wait for player spawning / any related interactions with the nether, until the world around the newly created portal has finished generating. Alternatively the default blocks in the nether should not be rock but air.

It's very frustrating to lose gear one spent a lot of time enchanting like this.