My 20255 is about 4 years old and has 163 hours on it. I noticed my rear tires have the middle tread with chunks missing from it. See pictures. I have a backhoe attached and a loader bucket. I seldom take the hoe off. Tractor is used for moving mulch, dirt etc and snow in winter.Anyone else every see this issue before, could it be a mfg issue or something else. Thanks.