Of the four pets that I booked into the resort (21 days), three of them had their health replenished, and the fourth did not. Yesterday I battled with the three that I was able to use. Needless to say, they suffered some damage. Today, their health is just the same as it was after they battled yesterday (over 24 hours ago, fyi). And my fourth, and MAIN fighter, is still at 0 HP.