I would think it is fixable. You're just gonna have to determine a way to put a dip in the fretboard instead of a hump.
Most dulcimers with a full length fretboard will keep relief, or a dip in the fretboard under tension due to string tension. The problem with this style fretboard is the partial length transfers the tension it's getting from the strings to the soundboard and sides and it will sometimes make the problem worse because the tension will tend to lift the rear end of the fretboard.
You'll likely need to pull the frets and use a hand plane or handheld sander to remove some material to make a dip, or some sandpaper and elbow grease and time.