I hope I understand how this is working. I think this is being caused by the spellbook, and isn't a bug in Simple5e.
If I use a spell to damage a PC token, the health bar is displayed. (I like this.)
But if I heal with the Simple5e 'Heal Damage' macro button, the health bar is not updated.
I think Simple5e doesn't use health bars as it doesn't use macros to target damage on other tokens.
This leaves my token with an indicator out of sync with my actual health. When I heal via the spelllbook it works fabulously, of course.
Aloha.