When there is an inventory screen, whatever item is being hovered over goes in the hotbar when you exit the screen.

Every time an inventory screen is open, any item you hover over when you exit the screen is now in your hotbar and it even crafts things if your hovering over a recipe book item. Some items don’t go into the hotbar for some reason and sometimes they do but most items go in the hotbar if you hover over them and exit.

Also it was happening to me before 1.21.100 and 1.21.94

How to reproduce:

• Enter an inventory screen.

• Hover over a block or item in a slot or in the recipe book or in the Creative menu.

• Exit the inventory screen.