My son's 2019 Ford Focus 1.5 diesel shows an EGR restricted flow light every 8000 miles. We clear this by dismantling and clearing a metal mesh filter which gets completely blocked with soot. This is located on the metal pipe as it comes out of the exhaust before it goes into the EGR and DPF. Once the filter is cleared it runs for another 8000 and happens again. Rinse and repeat.

Any suggestion what is causing this build up which seems more than the mesh filter is designed to cope with? The engine runs well and does five hour continuous runs on long journeys faultlessly. We have used fuel system and injector cleaners which improve running.