Steps to reproduce:

• Clone the 1.17 Example project
• Paste the following code in onInitialize

        ClientPickBlockApplyCallback.EVENT.register((player,result,stack)->{
            return player.getInventory().getStack(9);
        });

• Put an Item in to the top left slot of the inventory
• Middle-Mouse-Click something

Expected Result:

The item in the top left slot gets moved into your selected hotbar slot

Actual Result:

Nothing Happens

Fixed by #1498