EDIT: I have realized this is not a bug. I didn't chanage one of the NBT tags, sorry for the trouble.

What I expected to happen was
I would put on a chest plate that gives me 4 more hearts, and boots that give me 2 less hearts, and I would have a total of 12 hearts

What actually happened was
The first piece of armour I put on, the attributes carry over, then the next one overwrites all the attributes on the other pieces of armour.
Ex: Chest plate "+8 max health" put on first, total health: 14 hearts
Boots "-4 max heath" total health: 8 hearts
I testes this with all 4 attributes for items, the same result each time. Also when I took damage from a zombie it set the attributes I had from armour to my chest slot.

Steps to Reproduce:
1. Use NBT Explorer to add heath attributes to armour.
2. Put on the armour that gives you more health
3. Put on the armour that gives you less health
4. You will have the attributes the most recent armour piece gives you.
5. Take damage the attributes are set to the ones in the chest slot