If the relief is dead straight, with no back or forward bow, then I'd highly suggest using a straight edge and/or fret rocking underneath where the 1st string sits across the fretboard and see if the frets are raised at all. A very small variance such as 1/2 mm can cause buzzing on guitars with low action and no relief, as the clearance over the next fret is incredibly small, and strings vibrate in an elliptical pattern when struck.
It could be a resonance issue, but those don't just magically appear and disappear. Unless you're done something to the guitar to add a significant amount of mass, or remove a significant amount of mass, then the resonance of the guitar is the same as the day it was built. A quick test to rule out resonance is to attach a pair of heavy vice grips or a C-clamp to the headstock, this adds mass to the guitar. If the weird sound disappears or moves to another set of frets then it's a resonance issues.
Also, have you tried looking at your bridge saddle? Could there be a burr the string is vibrating against?
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