The bug

After attributing yourself with a higher max health then dying the previous row added becomes ghost hearts not regenerating or giving extra hearts.

How to reproduce

1. Make sure your health/hunger bars are full
2. Run these commands in order while in survival mode:

   /attribute @s minecraft:generic.max_health base set 40
   /attribute @s minecraft:generic.max_health base get
   

   notice the result is 40, and the health bar has doubled and will slowly regenerate a bit

1. Now run these commands:

   /kill @s
   /attribute @s minecraft:generic.max_health base get
   

   notice how it now says 20 and the hearts in the top row are now empty and refuse to regenerate