I've noticed that recently my health points decrease when I'm in a battle sometimes.
At the moment my HP is 574 but sometimes when I'm fighting it drops a few points - in the most recent case to 563. This seems to happen when I've been hit, so e.g. if I'm hit for 20 damage then the current total drops to 543 and the maximum shown to 563. When the fight is over the maximum jumps back to 574 and the current total increases by the same amount.
I can't think that there is any in-game reason for this, so is it a bug? If so, is it fixable - it would be annoying to die because of this effect!