This pertains only to QuickBooks on line. I have similar problem in desktop. I USED TO BE ABLE TO  edit the Email "form message"  when sending an invoice to customers for payment.   Recently- as on November, 2022, when I push the SEND button, QuickBooks automatically  reverts the message back to the standard default  message. I can no longer send a custom message out of QuickBooks.  This lack of flexibility and new protocol is horrible.  We are not BOTS- although QuickBooks apparently attempts to make us so.