Sorry no, I should of updated my previous post, as I tried to reproduce it after making the post but couldn't (including downing other charmed party members, about 10 different attempts against the next Fampyr I encountered). I had overwritten the save for that fight, so had to try reproducing it on another fight. Whatever the issue was it must be triggered by a combination that I can no longer reproduce (i.e. maybe he had another effect active etc...).
Maybe close this bug, and if I encounter it again I'll make a more detailed post including a save game.