How to recreate:

1. Have a health scoreboard, /scoreboard objectives add health health
2. Give yourself a item with negative MaxHealth, /replaceitem entity @s armor.head minecraft:leather_helmet{AttributeModifiers:[{AttrbuteName:"generic.maxHealth",Name:"generic.maxHealth",Amount:-2,Operation:0,UUIDLeast:137757,UUIDMost:964305,Slot:"head"}}{{}]} 1}}{{
3. You see that the health score dosn't go down or updates even when you regenrate or having some absorption
4. When you clear the helmet the health goes back to your original health