I have ADHD. I have had it my entire life. I was "diagnosed" with it when I was 14. I was therefore "undiagnosed" for 14 years.
Similarly, cars can have problems that a mechanic can't figure out how to fix. These are generally called undiagnosed problems, but they're still there.
Sure, but this person implied that they had been to doctors and the doctors disagreed with their own self-diagnosis. That's a very different thing than simply not having been diagnosed.
Similarly, cars can have problems that a mechanic can't figure out how to fix. These are generally called undiagnosed problems, but they're still there.