Of course they wouldn’t say it, generally, you’d just be round filed along with 90%+ of the other applicants and never know why.
Unless they’re entertainingly dumb, in which case they’ll bring screenshots. Which has happened.
Most companies don’t want drama with, around, or from employees. They want people who get what they need done, done.
Something comes up that looks like drama? Round file.
And good luck contesting it if they do this - courts generally give quite wide discretion to hiring decisions like this. It would need to be explicitly based on a protected class, which is quite narrow, AND they’d have to have documented it as such - which they’d have to be complete idiots to do.
Oh, and you’d have to know about, and find the documentation.
Do you think HR is going to investigate? The type of company that rejects you for something like that is just going to look at the thumbnail and not mention why they haven't hired you.