I can't say I've seen this behaviour (though, if I'm honest, it's likely it did happen and I just thought it was part of the game); but one thing I have noticed, which is really grating, is that enemies tend to swarm my character, even if there is a pair of tanks standing in their path, brandishing weapons and looking generally mean and tanky.
I haven't yet decided if they swarm my character because it's the "player" or if it's because she's basically a dedicated healer (Lifegiver/Priest (Eothas) with a number of AOE DoT/Buff spells) and the AI wants to get rid of the "biggest" threat -though I'm inclined to believe it's the former.