It doesn't seem to autocast on itself or me. I tried getting myself below 10% HP and still nothing. According to everything I read, it is suppose to autocast when you are below 50% HP.
I've had it both on autocast and a mouseover macro (shared with bandages, when a spirit beast isn't both my active pet, and alive) for the longest time now, so I haven't noticed the issue. I'll have to give it a test today.