Just as a minor addition, I had this happen a couple days back: my pet had snagged aggro on something while I was running towards a nexus rift. It died when I zoned through to the other side. No big deal, pets die, that's what they're for.
But when I resummoned it, it's hp was at the point they were at when I had zoned the into nexus. So I then had to let it heal back from around 30% before I could go hunting.
Was this an intentional effect, to keep people from hunting near a zone line, popping across to safely summon a new pet when the old was near death, then coming back to finish the mob before it regens?