It was while I was being attacked, yes, however I don't seem to have had this issue until recently. (Like.. 5.0+). I will see if I can try and get it on video, but I know i wasn't being interrupted as the skill goes on cooldown and the animation plays, so it's as if the heal just didn't land. I would like to report this bug, but, it's been difficult to reproduce and whenever I am recording it doesn't happen of course.