Re: Suddenly the 11th or 12th fret gave up on me
So one of the strings in your first (E) course frets out on the 12th fret, while the other doesn't?
The only thing that comes to mind is what you've suggested, either that the slight curvature ("relief") in the neck has lessened, bringing the 12th fret closer to the strings, or that the fret has risen in its slot. I assume by "bottom E string" you mean the first string that's located closer to the edge of the fretboard, so there also could be a slight twisting in the neck that raised the outer edge more than the rest of the fretboard, so that only one string of the first course was affected.
If it looks like the 12th fret's still firmly seated, and appears to project above the fretboard the same distance as other frets, check your neck geometry -- or have an experienced repairperson do it for you -- and see if the neck's moved or twisted at all. Is it only the 12th fret that seems "raised?" What about the 13th and up -- do they seem to fret true?
The other possibility is that your neck's "relief" may be affected by new, different strings, exerting less string tension and allowing the neck to flex "backwards" to some extent. Your problem may be solvable by a slight bridge height adjustment, but I'd get an expert to look at it.
Allen Hopkins
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