I don't use macro, but if the tank dies or becomes deselected from being too far or what have you, the heal will target myself.

If I'm using say like the gamepad and I don't confirm the target I'm healing, after the first heal, the target will snap back to the primary target. If no primary target selected, the game will just heal myself. I thought this was an intended mechanic, so I've been using it to target myself sometimes.