I spawned in with 1 person on my team and got to the first encounter area and it was empty so I thought we had just dropped in behind someone who was solo, even though I didn't have a name tag for a fireteam member ahead of me, and I went forward to catch up. Got into the underground area, still no enemies have spawned, went down to the section where the witch should be at the bottom of the stairs in the water room and was stuck in there. Another person joined my game and I went back up to the entrance of the underground area and it was sealed off now too.