Sounds to me by your description that the fingerboard has swollen a/t 12th fret. There's a tendency in older instruments of the mandolin family to need a good shaving of the fretboard every hundred years or so. :-) Losts of string tension w/o a lot of bracing or in cases of older/cheaper, but good sounding instruments no truss rods.

I play all the mandolin family instruments and it's been my experience that 'celtic' style, round hole examples have this problem more than the bluegrass style f-holes. Though that's not always the case.

One instrument I purchased is from the 1860s and looks like a large version of a celtic mandolin. The bridge is too low for single string work, but as a rhythm instrument in addtion to the guitar it just sings. I've strung it up as a mandola. To make it playable for single string I'd have to invest in a neck reset so I decided to live with it as it is and use it for what it can do.

Sorry to drift. Again the neck may be fine. My guess is the fingerboard. But, have someone who works on mandolin family instruments take a look. They'll be able to tell you what the problem is w/very little effort.