The Invoice ID at the top of this form is only a reference to find the Client ID. Therefore when you enter an Invoice ID at the top of this form, it is not actually stored. You may notice that when you type an Invoice ID for a Client that has two or more outstanding invoices, the Invoice ID will change to another Invoice ID for that Client. This behavior is by design.
Another tip - type Alt+I to go to the Invoice ID without using the mouse.