These issues are not uncommon.
One thing you may do, depending on how far from a post office you are, go into a post office and explain the issue. They have more info on their system than is shown on the USPS.com lookup. You've already opened a case for missing mail, which would be the next step.
Have you called the IRS about this document? If you haven't I would advise doing so. 1800-429-1040, call at 7am, your best chance of reaching a human. If they say they have it, problem solved
If they have no record of receiving it then I would send it again, include a cover letter indicating that it was originally sent on XX/XX/2025, you were told there is no record of it being received so you are resending it. Hopefully the agent will put in notes about the call in case the first one shows up in the meanwhile.
If mailing, send it certified mail with a return receipt requested. That seems to be the most thorough way of tracking sent mail.
It is also possible to fax the document to the IRS. The fax number is 844-249-8134, but if choosing this route please check the article on faxing the IRS to make sure they are still accepting faxes. Here's the link:
Form 3115
Then I would contact the USPS customer service and request a refund for the postage of the priority mail that did not arrive. You can do that by signing into your USPS account, which you have since you set up a lost mail look-up.
Best of luck getting this resolved.
