Happened to me as well. I had a good look at what happened:
I engage the whole group the first time, Servis was dealt with quickly, however instead of staying on the group he got up (he was on the ramparts at the moment) and started walking... in the air. When the whole group was dead he was too far out of range (and still walking in the air) to be able to do anything about it.
I've tried reloading multiple times and the only thing that managed to do is that he stopped showing up at all. Unfortunately the earliest save before the bug is hours before so I'm not really willing to waste all that time.