If you are aiming at an opponent, whatever skill other than buffers will be casted on to your opponent. In this case your heal is casted onto your opponent. You have 2 options. To constantly aim at yourself and just hover your mouse pointer over your opponent and cast skills or to switch targets between yourself and your opponent. I don't suppose there is an easy way out of this unless you put bot on to only heal you at certain %'s.
It's not a bug and necessarily bad planning. It's just logical.