Basically, any armor items with a modifier to increase your max health will work as normal, but when leaving the world and rejoining, any health you may have had past 20 HP will be lost.

What I expected to happen was...:
Health past the default maximum of 20 given by an attribute modifier (note: setting the base value of the player's max health above 20 does not do this, it resets only if an equipped item has a modifier that increases max health is the source of the extra hearts) should be saved when rejoining the world

What actually happened was...:
After healing to the maximum allowed with the modifier and leaving the world, rejoining resets your health back to 20. The max health capacity is then set back to whatever the modifier adds to the base value, leaving that many empty hearts.

Steps to Reproduce:
1. Equip any armor piece with a max health modifier which adds the set value to the player's max health. In my case, I had a pair of leggings with a max health modifier which adds 20 to the base amount, resulting in 40 HP (20 hearts) total, but this isn't limited to leggings, and isn't limited to this exact amount of HP. You can do this with any armor piece that has this kind of modifier, as long as it results in an increased amount of health.
2. Allow your HP to regenerate above 20.
3. Leave the world and rejoin.