I'm not sure of the exact conditions but I've seen this 3 times since the last update. Yoran will stop all function under certain conditions. I mean this literally. He won't cast. He won't do his little "run around to find a safe spot" thing either. He won't use Dagan. He won't do anything, and I mean, exactly that. Anything. He is not asleep/silenced, he will still follow the party around.
No I won't fill out a bug report.