I really believe that a lot of the reports of cracking fur have as much to do with the time of year and weather as it does with the glider's diet.
With the approach to winter MANY animal's fur goes through a growth change and they develop a thicker under coat of very fine fur that helps keep them warm during the winter months. Dogs and Cats in our care develop thicker winter coats even though they live in our climate controlled homes year round.
Why wouldn't a glider's fur also become thicker. As the undercoat gets thicker, it will naturally look as though the fur is splitting and 'cracking' in places.
Even gliders on one of the recognized glider diets may show cracking this time of year, especially with the extreme cold temperatures many folks are currently having. Add the heat you provide in your home to dry out the air more than the winter weather already has and it will just make the cracking more noticeable.
If you are feeding your glider's a good staple diet along with a wide variety of fruits and vegetables the cracking is probably from the seasonal changes, weather and dry heated air combined.