His post earned a reply from John Yang, who explained the problem; the Ancients weren’t healing as they were supposed to, a problem that is fixed in the next PTR patch to 2.3, due this week.
John Yang: There was a bug with pet life regeneration where player pets were receiving significantly less life regen than intended. This will be fixed with the upcoming 2.3 PTR Patch 1 scheduled for this week.
Call of the Ancients is the most visible example because the Together as One rune redirects a lot of damage to them. In their case, they are intended to regenerate about ~5-6% of max life per second baseline, but previously were only regenerating about 1% per second.
It sounds like most/all pets will seem a lot hardier from now on. Is every mortal pet about to go up 5-6x in their regen rate, or just the Ancients?