The "max_health" modifier makes very strange changes when used. For example, in a map, the item "Shadow Chestplate" was created, adding 10 hitpoints (5 Hearts) when worn. Another item, when worn, added 4 hitpoints (2 Hearts). When wearing either both of them, it would replace the health with the armor most recently equipped

Further Explanation:

Equipped armor and gained 5 Hearts. When equipping another one (adding 2 Hearts) after that, it would not give a total of 7 Hearts, but rather replace the 5 Hearts with 2 Hearts.

Not sure if this only applied to my specific world, will further test this after submission.

The commands used were very, very long, but I did recreate it using other commands:

/give @p minecraft:leather_chestplate{display:,AttributeModifiers:[{AttributeName:"generic.max_health",Name:"generic.max_health",Amount:10,Operation:0,UUID:[I;42853,1689024593,-201178,-1559272105],Slot:"chest"}]} 1

and secondly:

/give @p minecraft:leather_helmet{display:,AttributeModifiers:[{AttributeName:"generic.max_health",Name:"generic.max_health",Amount:6,Operation:0,UUID:[I;42853,1689024593,-201178,-1559272105],Slot:"head"}]} 1

If this was an intentional game design, then I'm sorry for the waste of time.