This is  EXACTLY what happened to our invoices. We did not make any changes in QB but one day the formatting changed to the red lines and QB file name, which is not our dba name, and customers started having trouble with the pay now link. Our customers thought it was a scam email so AR has been a challenge.

As I mentioned, the only work around we have found is we moved away from QB mail in forms preferences and the invoices now come from my email address, it could not be a shared mailbox. Not ideal in case I am out of the office.