Players spawn with 20 health, not with maximum health.

I made an armor that takes 10 health then I died with the armor in my hand (gamerule keepInventory was true) and respawned with it in my hand too, and my health was 20!
I don't know if it's a feature or a bug.
Also, if I wear the armor my maximum health doesn't change, only if I hold it the max health changes.