If a player has the Health Boost enchantment and have HP>20 (10 hearts), after restarting the game (including returning to the main menu and load the world again), the HP will be reset to 20 instead of keeping the value before.

Which mods cause this doesn't happen with Spice of Life or Twilight Forest you can even check using my pack Fiction Friction I am also happy to op you. Did you add debugify which fixes a known vanilla bug? What about attribute fix? You need at least that for the game to stop resetting you too otherwise you will get a NaNHealth issue.

OK, it's a vanilla bug. (MC-17876)
It can be fixed by installing Max Health Fix by DarkhaxDev.