That is absolutely bizarre.
I did have a friend many years ago whose job application was ripped up through the mail. He wasn't a friend at the time, just someone applying for a job, who would be working alongside myself. My boss showed me the mangled application and together we deciphered enough of it to decide to bring him in for an interview. He got the job, we became good friends and remained so this past 40 years.
I'm just wondering if your page to the IRS had a similar mishap, torn apart by some sorting machine, but even so I would have thought the IRS would have included some explanation when they returned it to you.
I think I might resubmit the page, along with the ripped page with a note to the effect that it looked like the post office had mangled the original and you hope that this one makes it okay.