Non-flagged players are taking damage when being shot by flagged players

I was roaming around ~8:00pm EST on Casual and ran into another player. I was unflagged and was being shot at by mr.boost.moore, the first shot caused mr.boost.moore to become flagged. I also took damage from 100 - 76.7 from this first shot, he then fired 2 more shots at me, each time I took damage. I got away and into a safe location where I attempted to unflag (figuring I hadn't noticed I had been flagged), instead I wound up flagged for pvp the instant I entered the /pvp command.