Kids often find family secrets when it is time to get a driver’s license or apply for financial aid. Not all are terrible stories of kidnapping or worse, but sometimes that paperwork was never completed. I guy I worked with from a wealthy family (prep schools, Princeton) had a sister who wasn’t documented. His parents had just brought her home from Mexico when she was a baby and never completed paperwork to adopt her or get her citizenship. It was never an issue until she was an adult. They lived in NYC so she didn’t drive, they didn’t need FA for school, and I guess they never traveled to anywhere she needed a passport. Of course, it was then a big mess when she needed documentation.
Internationally adopted children often find paperwork mistakes when they apply for FA. The SS status has never been changed to ‘citizen’ or the adoption was not properly finalized in the USA.
One thing that isn’t clear in the above stories is that the Reddit poster says it was his father who told him that he couldn’t give the tax info because of the fraud. If the son knew that, why proceed with the applications? Why use the SSN for applications (you don’t have to unless you are applying for FA)?