I've found that healers don't relocate at all just to heal people. If the enemy they are targeting is within their range then they won't move. If someone within their range needs healing then they will heal them (usually). But if you step out of their range then they will completely ignore you and just shoot the mob.
Yeah, I agree with this. One of the few real flaws in their AI IMO. Sometimes I have to pull a fight back to them so I don't inconvenience them to have to move to the fight to heal. Silly companions.