3 of our drivers have had people try to claim cracked windshields from rocks that were kicked up. ALL 3 of them were denied and had to handle it through their own insurance. I don't know about the mudflap thing as all 8 drivers have them. Maybe that is why the people were denied claim.
If a truck has mudflaps and a rock is thrown up, the truck isn't liable for the damages caused. If a truck doesn't have mudflaps and a rock is thrown up, the companies insurance is liable for the damages.