Let H be an item that gives an attribute A, when held in "any" slot (the main/offhand and armor slots).

Switching H from offhand to mainhand, or armor slots to main/offhand disables A (unexpected behaviour), and switching H from mainhand to offhand, or main/offhand to armor slots does not disable A (expected behaviour).

Item to reproduce:
{{/give @s minecraft:leather_helmet[minecraft:attribute_modifiers={modifiers:[

{name:"armor",amount:1.0d,type:"minecraft:generic.armor",uuid:[I; 0, 0, 0, 0],slot:"any",operation:"add_value"}

]}]}}