If you use a command to give yourself a single piece of armor with a max health attribute on it like "give @a netherite_chestplate[attribute_modifiers={modifiers:[

{type:"generic.max_health",amount:2,slot:chest,id:"generic.max_health",uuid:[I;-12466,14410,11358,-28820],operation:add_value}

]}]" it will work fine. But if you have two pieces, only the one which you equipped last will add its effects to the player even if the value is lower. In older versions of the game the attributes from both armor pieces would be applied to the player unlike 1.21. Also if you have 2 pieces of armor with the attribute from an older version of the game and updated the world, it would work normally. (The bug only happens when you create the armor with commands in the current version (1.21).)