Players with modified max health naturally regenerate to 20 health even when their max health attribute is not 20 (above and below).

Instant health effect always heals up to 20 health even if the player's max health is set to another value.

The regeneration effect always heals up to 20 health even when the player's max health is set to another value.

In summary, max health attribute really only changes how many hearts your game displays.