My understanding is that the the hearts are adjusted so max health is always displayed as 10 hearts. Check the attribute minecraft:generic.max_health when you equip a set or:
Give yourself an armor set with +20 bonus health
Equip the armor, your health should be displayed as 5/10 hearts (because max health has doubled)
/heal or wait to regenerate health
Unequip armor , health should be displayed as 20 hearts
Describe the bug The effect Bonus Health doesn't work properly
To Reproduce Steps to reproduce the behavior:
Expected behavior Well, obviously get more hearts based on what's written in the configuration
Screenshots N/A
Server Information (please complete the following information):
Additional context N/A