simply Scale max healt option not working later sleep, i have a mod to can add you more hearts, so it add me 2 hearts in the hearts bar, to allow you to use the additional hearts with first aid I have activated Scale max healt, all work normal, FA add me the hearts divided from the head, body, legs, etc., let's pretend we have 2 lines of hearts in minecraft bar, you will have double hearts for each body part on First Aid, (ex: the head goes from 2 hearts to 4) so I decide to use bandages to have max hearts, the problem is that when you go to sleep, when you wake up you will see only 1 bar of hearts, after a few seconds you will return to having 2 bars of hearts, but, the second bar will be emptied, so to have the maximum life again I would have to reuse the bandages, in a nutshell using mods that add more hearts to you, after you go to sleep the additional hearts will have to be regenerated (fun fact: if you have 1 full bar, and the additional empty bar will give you slowness, mining fatigue etc)
Im using First aid 1.6.16 with minecraft 1.12.2

I have the same problem