The bug
The /item modify command makes armor and offhand play their equip sound when you modify their components.
How to reproduce
Equip a chestplate
→ ✔ The armor equip sound is played
Run the following command:
/item modify entity @s armor.chest {function:"set_damage",damage:-0.1,add:true}→ ❌ The armor equip sound is played again
Can confirm that this also applies to Elytra, which will make shuffling sounds when modified.