Pick block on items outside the hotbar visually replaces armor/crafting slot with duplicate of the stack that was picked

To reproduce:

1. /gamemode 0
2. /clear
3. /replaceitem entity @p slot.inventory.26 grass
4. Pick block on grass
5. Open your inventory, look at your armor/crafting slots

Which slot is replaced depends on which hotbar slot you have selected:

1: crafting output
2-5: crafting input
6-9: armor

Video: https://www.youtube.com/watch?v=746_NNH_UfA