I've been having this same issue for a while. It usually only happened once in a while so I thought the problem was with me but earlier during a Haukke run, I tried to heal the tank 3 times and all 3 times it healed me instead and nearly got him killed. It wasn't distance or line of sight since we were in a small room with no obstacles around.

I'm thinking of just always keeping my selection on the tank and adding a heal target after mo heal in case mo ever bugs out again.