Okay. This bug is kind of complex, so here goes.

I created two pieces of armor, a leather helmet and a leather chestplate.
The leather helmet had +1 max health (so half a heart) with the attributename:generic.maxhealth.

The leather chestplate had +4 max health.

In the First picture, i have nothing on. I have the standard 20 health (so 10 hearts) as any other player.

The second picture shows me only wearing the helmet. The max hp increases by +1. No harm, no foul.

The Third picture shows me wearing both the helmet and the chestplate. The max hp increases by 1 + 4 = 5 (2.5 hearts). Nothing wrong here.

Here is what gets interesting. As shown in the Fourth Photo, If I take off the chestplate before the helmet, the amount of bonus health I should be getting is +1 (half a heart). But instead, even with only a +1 health boost on my equipment, I get a +2 (1 heart) health boost.

These are the commands I used for the equipment:

/give @p minecraft:leather_helmet 1 0 {AttributeModifiers:[{AttributeName:"generic.maxHealth",Name:"generic.maxHealth",Amount:1,Operation:0,UUIDLeast:894654,UUIDMost:2872}]}

/give @p minecraft:leather_chestplate 1 0 {AttributeModifiers:[{AttributeName:"generic.maxHealth",Name:"generic.maxHealth",Amount:4,Operation:0,UUIDLeast:89463,UUIDMost:2868}]}

Please help me with this. Is this a bug, an intended design, or a flaw in the command?

Thank you.

P.S. I am also aware that many players have reported a bug where players will lose all their hearts gained from armor health boost attributes if they log off and re log on. Is this still being looked into?