Games : Are Area Effects Like Flame Cloak Supposed to Harm Allies?

Mercer Frey is the worst thief and worst "follower" ever, but his behavior on the Speaking With Silence quest reminded me to ask about area effects. My preferred tactic when in tombs is to lure the undead out one at a time, then pick them off from a distance. Frey, however, immediately ran in, screaming, and draws out six or so Deathlords at a time. So I switched to casting Flame Cloak and then rushing in with Dawnbreaker.

But occasionally I got a message "Mercer Frey resisted flames". And when Dawnbreaker does its undead burst effect, sometimes Frey would be damaged. I thought:

1. Dawnbreaker's burst effect is only supposed to affect undead, and


2. the Cloak spells aren't supposed to harm allies. I've seen it happen every once in a while before, such as when vampires attack the streets of Whiterun. Every once in a while, I get "Whiterun Guard resisted flames".

So, is this a known bug? Or do Flame Cloak, Dawnbreaker, and other area effects work differently than I thought? I'm afraid to try Mayhem while travelling with a follower, now...