Still don't see this happening in instances I've been in. Maybe it has something to do with sub-targeting the tank, you know, when the targeting ring/circle appear on-screen when you're targeting a player/enemy/object – this doesn't display the target's information in the target bar – but you already a have main target – this target's information is displayed in your target bar.

This could be the problem, but don't quote me on that as I've not succeeded in replicating this issue to really see what's going on.

I do, however, always make sure that whoever I'm healing is confirmed to be my main target. And I never have an issue healing doing this.