Typically mana only dips if:
a) You're having to convert all your aetherflow stacks into sacred soil/lustrate (None to use on energy drain)
b) You're forgetting to burn the stacks and flip aetherflow once it comes off CD.
c) You're having to keep rezzing people

I've never actually been in a situation where healing alone has completely depleted my mp reserve (Invariably in tandem with lots of players dying repeatedly). That said, as long as you *had* mp left, it's irrelevant - heals are still flowing. It sounds like you simply had a bad tank in that instance; or a bad team who forced you to keep burning mp on rezes. Either way I don't think the fault lay with you.