I'm not entirely sure this is a bug, but considering barbarians had no qualms about killing my apostles, I think it is. Basically, I was at war with Arabia, and I could freely move my apostles through their border without any fear of attack from military units (their apostles still attacked mine). Their units could surround me and it wouldn't matter, they didn't see my apostles as an enemy. Also, another bug I found, I made peace with two city states during that war while Arabia was their suzeran. The city states were not locked at war.