The bug

Holding a stack of (yarn) ArmorItem and right clicking it equips the whole stack, while opening the inventory and clicking with the item on an armor slot equips only one.

Steps to reproduce

1. Get a stack of armor equippable items:

   /summon item ~ ~ ~ {Item:{id:diamond_chestplate,Count:64}}

2. First, open your inventory, and click an armor slot with the item.
   → Observe that only one gets inserted and only one can be inserted.
3. Run the command again.
4. Put the stack in your main hand and right click.
   → Observe that the whole stack gets inserted.

This can be fixed by either equipping one when right click equipping or equipping the whole stack when clicking, thus bypassing the "only one inserted" when clicking on the slot