There's a transition in the Undervault, which, when descending deeper, doesn't seem to work. It's the one in the center of the area, in one of those circular descents.
This is likely the same transition that I died on because it was broken and I could not return back up. Could a DM please check the transitions on the Undervault, going down and back up. I know it's time consuming, but it looks like Kontenku found the same bug my character found two weeks ago.
The bugged/broken transition is located (roughly) in the centre of the area on one of the circular descents/ascents.
Pwweaase :)
Not exactly your situation Somniis, but this issue should be resolved.