"Ghost" tone on high E-string?

[Jefferson]

Relatively new to nylon stringed instruments, but I'm hoping someone knowledgeable can help me understand what I am hearing and diagnose the issue. Essentially, my high E string (when fretted) doesn't seem very crisp/clear on some frets - not string buzz per-se, more like a slightly flattened note barely audible behind the note I am trying to play. Unfretted, the string rings clearly.

I'm using basic Pro-Arte regular tension strings on a 60's Gibson CO. The strings are about 2 weeks old. I don't note any fretwear on the guitar, or anything at the nut or saddle that could be causing this and, as I said, unfretted, I don't hear it. It's noticeable on every fretted note up the board, though it seems to be worse between the 3rd and 7th frets.

Also, I don't hear the ghost tone on the G or B strings which are also unwound on this set.

Any clues? thanks!