I've had sort of an opposite experience. A guitar I bought from a dealer in Las Vegas had a cracked ebony fretboard in the upper registers about 2.5 inches long. Since moving to Ohio the crack has closed up and you can barely see it now.
Luckily enough, the crack is exactly under the G string. My playing room has stayed at about 45% humidity all winter and the crack still looks great/barely noticeable. Fret Dr. is what I've used on the ebony, is that recommended?