I was working with commands, and when I used this command: /execute as @a if data entity @s Inventory[\{Slot:9b, id:"minecraft:redstone",Count:1b}] run effect give @s health_boost 1 1 true I would see the empty hearts appear (the hearts that appear when you have less health than your maximum) but I would immediately take damage and could not achieve a health higher than the normal 20. The command is supposed to give you health boost 2 when you have 1 redstone dust in slot 9b. I think this issue is linked to the attribute generic.max.health because if you change that attribute to say 6, then the same thing happens but this time you cannot regenerate your health past 6. The regeneration is coming from full hunger, and not the mob effect.

To reproduce this:

1. place a repeating command block set to always active
2. use the command above in bold
3. put one redstone dust in slot 9b
4. make sure you have full hunger.