Hello There.
After the 1.16.5 release, this glitch annoys me with Custom Max Health armors and using /attribute for base stats and modifiers.
the default max health is 20, so if we set it to 40 or use a custom armor to higher it up,
the max health of the player will be 40 or the value that we have selected and the player will start regenerating.
but if we set this back to 20 or lower the last value the player health will not change and the player needs to take damage to reset the second value that we have selected!

before 1.17 and higher, I used to play 1.16.5 with some custom health data pack items and armor and I never had this problem!