powerapp – Why does PATCH keep adding rows instead of updating the collection?

I’ve got a gallery on the screen with a text input field. When the value of the input field changes, I want to update the selected item with the new value but PATCH just keeps adding rows to the collection. I’ve got similar code on other screens that’s working properly.

Patch(colAvailableItems, galNewOrder.Selected, {Qty: Value(txtNewOrderQty.Text)} );

Any ideas?