Sertao - Oil non return valve

On my Sertao with 13500km the oil does not remain in the oil tank when the engine is switched off. It will all drain after a few hours. It just had a service at BMW and according to them they have not heard of this issue. They also say that it is not something to worry about. I mentioned to them that on F650.co.uk the issue is discussed and the drainage is due to a faulty non return valve  which is behind the oil filter in the filter casing. They say the valves are there to bleed the oil system.
Is this something to worry about and has anyone fixed this issue with replacing the non return valve?