I believe the interface between the ridges on the belt and the nylon fabric (which gives the tension strength) is not strong.
There is no fibre linkage or weave between the ridged fibres and the nylon. Hence the belts are poorly designed for transmitting load beween the belt teeth and the main nylon rings.
As nylon has little friction it is no surprise that the teeth fibres and the nylon seperate. This is compounded by the constant flexing in different directions round the pulleys and drive shaft.
Thus the powder like stuff in your housing is the bits of ridge that are slowly coming off.
Eventually a whole ridge comes off and then the nylon will melt and cut on the sprockets very quickly without the teeth fibre covering.
I have no idea what to do other than put a spray on the teeth so as to stop it being torn up
