I don't know whether anyone else has experienced this recently but this morning I went to check my home page. I had two "unread" messages. Hmm, interesting, I haven't posted anything recent and haven't had time to leave many comments recently, so I wonder what those could be.
I open up the message list. No new messages.
I scroll back, and back, and back, and back, and... 2017, someone sent me a "Thank you" about a comment that I had left on one of their images. Yes, they sent it (and I read it) 6 years ago.
I keep scrolling back, and back, and back... Ah, a general discussion with another member about some photographic subjects... from 2012. Which I do recall reading, 11 years ago.
So, why these, and only these, have suddenly been marked as "unread"? I have no idea.