This has happened many times for me. It just reminded me again when I was playing my hunter yesterday...
Pet's at 100% hp.
I mount up, flew somewhere, dismount and attack a mob.
Pet comes out with 50% hp.
Seriously, wtf? 4/10 times when I dismount, my pet's at half life. I'm 100% sure the pet did not take damage previously.
Is this a known bug? How to fix it?
And another thing... I have Roar of Recovery, Growl, Ravage and Roar of Sacrifice on the pet's action bars. I've disabled Cower. But again, like the health bar, sometimes after I dismount, Cower goes back into auto cast. Why? Its bloody God damn annoying.
The pet health thing has always happened to me also, some say it is because the server takes a while to catch up and apply your stats to the pet, but I don't know, if you wait a few seconds before getting into combat, then it is fine.
Pet abilities need to be on the pet bar to be taken off auto-cast. If they are not on your pet bar, they will flick back on to auto-cast again.
What happens is, when you first summon the pet, it starts off with his own life at that level, then it takes into account your stats, thus increasing its life, then it heals the pet to full. When you enter combat too quickly, it'll increase its life, but it won't heal the pet to full because you're in combat
The cower/growl auto cast bug has been a bug for ages