Since one of the faces is rotating the solid particle follows a circular path until it is expelled out of the inside diameter of the lapped faces. This happens frequently in boiler feed water applications. Chemical attack of the carbon. Something in the product or the flush is attacking the carbon filler. You are trying to seal an oxidizing agent.
Oxidizers attack all forms of carbon including the unfilled type.
The carbon combines with the oxygen to form either carbon monoxide or carbon dioxide. You are trying to seal a halogen like chlorine, fluorine , bromine, astintine or iodine. Some forms of de-ionized water will pit and corrode carbon faces Cracked or damaged carbon face. The product is solidifying between the faces. Carbons are strong in compression but weak in tension or shear.