Getting summoned breaks pet guard
 

Not sure if this is a feature or a bug ---> But when you are summoned by a level 51+ mob, it breaks /pet guard and causes pet to follow.

Are you sure the issue is guard ending? If you get summoned, the pet will attack the mob, so it's going to go onto the target unless you're spamming back

No, what I mean is it breaks /pet guard. So the pet will no longer return to a guard spot after encounter, and will start to follow.