ive seen 2 different variations so far, the first one is that the game just crashes entirely (idk what effect it has in multi player i was playing solo) and the other is that grendel is seemingly pushed through the floor and falls seemingly to the bottom of the skybox before being teleported back into the tile but with no animations, in both cases mentioned i was playing on the stickney node on phobos
im not sure if its manias switch teleport or the melee lift that breaks things, if i had to guess id say probably the lift
steps to recreate:
pick grendel,
go to steel path (stickney on phobos in my case)
wait for mania
go into pulverise and roll around on its face
get sent to the shadow relm