Forge version: 35.1.28
Futurepack version: 21.0.4

When I teleported into a new dungeon, I started falling out of the world after the dungeon structure generated.

Turns out the bottom of the dungeon was cut off by the Y=0 limit.

there are already checks against that.. how tall is the dungeon does it reach Y 255 ?

The highest point of the dungeon is approximately at Y=127.

If you think it will be helpful, I can upload the world somewhere.