I was playing around with the new custom Enchantments in Datapacks and found this little issue, when applying any type of Max health attribute to Armor via the Attribute section
Example:
/give @p netherite_chestplate[attribute_modifiers=[\{id:"max_health",type:"generic.max_health",amount:10,operation:"add_value",slot:"chest"}]] 1)
or
See Attachment (.json file belongs into datapacks/Namespace/data/minecraft/enchantment, dunno if the id has to change as well)
You gain the stats, but when exiting and rejoining the World they reset, which is not true for the Player own Attributes
/attribute @s minecraft:generic.max_health base set 40

I didn't directly find this while browsing the issues on here, if its a double I'm sorry... I have tried this with and without Mods and even on a clean install