How to reproduce:

1. “/give @s diamond_chestplate{AttributeModifiers:[

{Name:a,AttributeName:generic.armor, Amount:8,Operation:0,UUIDLeast:1,UUIDMost:1,Slot:chest}

]}”

2. “/give @s diamond_boots{AttributeModifiers:[

{Name:a,AttributeName:generic.armor, Amount:4,Operation:0,UUIDLeast:1,UUIDMost:1,Slot:feet}

]}”

3. Wear them on and notice the armor bar shows 2 armor bars OR the armor bar shows 4 armor bars until the player attacks something(or something to update the player) then turns into 2 armor bars.