KILL your buddy?
While doing a run in the Pig City, I activated my buddy (Grey Slam Grimlock) and he fought alongside me (Windblade) until we got to the pit- since he was ahead of me, he FELL in (accidently) BEFORE I hit the "switch" (the thing that activates the holographic bridge), so he went down and I ran across the bridge like you are supposed to! The buddy "button" in the bottom left VANISHED as soon as he fell in and stayed like that until the run ended, where he entered Astrotrain with me as it showed the Pigs and Coins obtained, so how did he come back?
(The run was one of those you use Gems to unlock and gives you large coin rewards)