My guess would be the problem is a leak in the heat exchanger in the bottom of the radiator. When warmed up, there is greater pressure in the radiator than there is in the coolant lines and exchanger so the radiator solution enters the exchanger but trans fluid does not enter the radiator. The antifreeze will strip the material off of the transmission clutches and this may have already occurred because of what you are seeing in the filter. I had the above happen in the black low mileage 300C convertible that appears in the 300 video. In that case it was acids in the radiator and lack of use over 18 years of storage. Wayne Buick 58 Roadmaster sedan