Ken - thanks for that info. Much appreciated!
Well certainly grain orientation and timber cut are important factors in preventing wood from warping, as are properly dried woods and proper glue joints.
You want a quarter-sawn fretboard with the grain running vertically from the plane of the top, not parallel to the top.
And you want a properly made glue joint to a quarter-sawn fingerboard with the grain running at right angles to that of the fretboard.
The resultant veneer with grains running at right angles to each other is a standard 'recipe' to prevent warpage
After the sandwich dries, then the arches are sanded or sawn or routed into the assembly. Finally the fret slots are cut, frets installed, and the completed fretboard is then glued to the top before that is glued to the rest of the carcass.
Flat-sawn arched fretboards without a fingerboard, may indeed warp, but that's not what I consider a "properly constructed" arched fretboard instrument.
I'm going to to disagree with Ken here. I haven't owned scalloped-fretboard instruments long enough to notice any issues, but I can think of three people I know, very well respected in the dulcimer world, who have been playing instruments with and without scalloped fretboards longer than I've been alive, and it is their opinion that over time, those scalloped fretboards do (and in the case of some builders, WILL) warp. And they each told me that independently, over the years I've known them. In fact, it's not even an opinion; two of them have shown me examples.