I got a similar response from USPS, they said that most tax returns are "accountable mail" that has to get signed for by tax agents directly at the USPS sorting facility, however in my case:
"Your item appears to have slipped through Postal automation and in with the regular First Class Mail without receiving its finalized delivered scan. When items slip through, however, there will not be a delivery record we can provide you. We apologize for the inconvenience this causes. Our employees attempt to cull out the accountable mail from the regular First Class Mail, however, some are missed.
Please retain your mailing receipt as you may use your mailing receipt as proof of mailing. We advise that if your item has no other scans after (30) days from the time of mailing with receipt, you are entitled to a refund from any Post Office for services not rendered, which in this case will be your additional Certified Fees Paid for the failed delivery scan."
And the email from USPS goes on to give me a phone number to call about confirming receipt of my tax return (must happen often if they have a number on file). Check my refund status still shows no record of my filing a 2019 return.
