I /bug'd my experiences yesterday in Nek. We were pulling fairies and they come in pairs. I'd mez the other guy, while the group whacked the lead fairy. After first fairy was dead, gruop would break mez on second one. Then he'd just fly away. You could root him, stun him, gaze him, everything. But he's still just fly away. I'm not sure if Mez is the one doing that. I've also seen the bug where if you Fascinate a mob and refresh it with Mesmerize, no Mez class spell will work anymore. /bug'd that as well.
Some people can proc a fear. Stuff in Nek does this and I think it is tied either to shaman wraith form or possibly some sort of AI that makes them run off if their buddies are dead etc. Its very easy to kill these guys, just start chain rooting and bomb away at em.
Our shammy wasn't in "evil beaver" form. We'd have the main tank break mez and the little dork would just fly off. Kinda funny watching a full group literally throw the kitchen sink at a fleeing mob to kill it. Both tanks throwing axes, me nuking, defiler nuking.
Some what off topic, but yes the faries will flee after being mezzed and their partner killed, however I haven't and it doesn't sound like you have done enough testing to know if this is a bug associated with mezz or not. Beavers on torrent isle in nektulos will also run after several companions have been killed, mezz regardless.
