Attribute modifiers don't stack properly in the snapshots.

I was making custom items with commands when I realized certain attribute modifiers don't stack.

What I expected to happen was...:
I made two pieces of armor that should have each increased your max health by 1 additional heart (2 health), totaling to 2 additional hearts (4 health).

What actually happened was...:
I only received 1 addition heart (2 health) even while wearing both pieces.

Steps to Reproduce:

run the following commands to gain these items:

/give @p iron_leggings[attribute_modifiers=[\{id:"max_health",type:"generic.max_health",amount:2,operation:"add_value",slot:"legs"}]] 1

/give @p iron_helmet[attribute_modifiers=[\{id:"max_health",type:"generic.max_health",amount:2,operation:"add_value",slot:"head"}]] 1

wear both in survival or adventure mode.