I was making custom items with commands when I discovered that specific attributes (like the max health and movement speed attributes) didn't stack.
I made two pieces of armor that were originally supposed to increase your max health by 5 additional hearts, the first piece giving 2 additional hearts and the second giving 3.
Instead, I only got the effects from the most recently worn armor piece. If I wore the first piece last, I would only get its additional hearts while the other piece gave nothing, and vice versa. 
I checked the UUIDs and they are not the same.

Commands used for the items:
/give @p leather_helmet[attribute_modifiers=[

{id:"max_health",type:"generic.max_health",amount:4,operation:"add_value",slot:"head"}

]] 1
/give @p leather_boots[attribute_modifiers=[

{id:"max_health",type:"generic.max_health",amount:6,operation:"add_value",slot:"feet"}

]] 1
These items were both worn in survival.