There I was running around by myself on Venus collecting spirit blooms for a weapon upgrade with my secondary character when I come up on the beginning of a raid. Five guys are trying to open the gate. So... I went to help the single player who was having a hard time. I killed 4-5 Praetorians, and the gate opens.
I chase after the five, and make it through the big gate. The five were a short ways ahead of me, but when I rounded the corner where the first loot chest is normally located they all disappeared. And... the path forward becomes blocked. I couldn't go back either. The gate is now closed. Spent some time exploring and jumping to different ledges. Eventually went to orbit.