I am not even sure how this happened. The enemies quite clearly went to the location of my character, the leader of the party in my MP game, and kind of just surrounded him. They clearly couldn't see and hit him, they just circled him and trapped him. I can't tell if it is a bug, but I can't think of how it happened.
Anyway, I don't have a good save for testing if you ask. I didn't think of it at the time. Did take a picture of the situation, though.