I was certain I fixed this in 1.6.6 but I may have overlooked something. Do those characters have equipment that increases their Max HP by any chance? That was the condition for it before, but it was upon reloading a save that it would reset their HP. This is happening anytime you save now?
The characters just had the basic starting gear, but I tried removing all gear from them and repeating the process and it still happens with the same values. It isn't from loading the game; just saving the game is doing it. Actually, after checking further, going into the save menu at all triggers it, even if I back out of the menu without saving the game.