It seems like this only happens when you try spawn and enter a second necramech before your current necramech dies. In my experience if i tried leaving the necramech to exit the vault and spawned another on the surface after a while my frame would vanish. But i've just run a few missions where I let my necramech die before leaving the vault and spawning another and my frame hasn't vanished during play yet. Sort of a work around to still get necramech xp until they patch this bug
When trying to throw the bait at the 2nd tier vault infested door the bait disappears if not thrown by the host. Was in a lobby hosted by someone else and I threw the bait at both doors and the 2nd one vanished as soon as it was thrown. Started another lobby immediately after where I was the host and I was able to throw the bait at all 3 doors without it vanishing. Prior to these two instances I another lobby where someone said that nothing happened when they threw the bait at the 2nd door though I don't recall whether they were the host or not (I suspect they weren't though).