I've never used belt dressing on my Grasshopper, 300 hrs. in 3 years sounds somewhat short life. I get more than double the hours out of my deck belt.
Growing up on the farm in the 60's we used belt dressing on 10" flat belts for tackiness to control slipping.
I wonder if you could be getting too much grip causing too much stress thus cracking and loosing chunks of the belt?