Playing with my husband in multiplayer, we were in an antiquarian swamp, just running from one infestation to the next and jumping while running. Well, I jumped and instead of hitting the swamp I ended up under the terrain. I wasn't able to use the unstuck button and had to give up and respawn. Very sad.
Just happened to me too, and also Antiquarian swamp infestation site, was gliding to it and as I dropped there was no floor under the water and i kept going, ended up back at the Lodestar, and survivor recruit erased from existence in the process