Expected Behavior:
Attribute modifiers on items ('attribute_modifiers' component) take effect immediately when put in the correct slot, and immediately removed when taken out of the correct slot.

Actual Behavior:
Attribute modifiers have a 1-2 tick delay before taking effect when put in the correct slot, aswell as having a 1-2 tick delay when being taken out (seemingly inconsistently, but unverified).
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To reproduce:
Give yourself any item with an 'attribute_modifiers' component and put it in the correct slot, observe that the attribute is not modified for 1-2 ticks.

This is very easily observed with an place-able item that sets 'player.block_interaction_range' to 0 via attribute modifier on the 'mainhand' slot. This item should not be place-able, however, if you switch to the item and right click in quick enough succession, you still can place the item.