I have a porsche cayenne 2013 diesel 3.0 tdi and i have had a problem for a few years now and i cant find a solution. 2 years ago i was driving around 130kmh and i had on the screen it said low coolant so i stopped on the side and waited for it to cool off and topped off the coolant. I continued and ever since i have had the same problem mostly when im on a long driving trip. My issue is that after the car cools off even after a day of sitting when i open the coolant tank there is pressure in the tank and it is as if the engine has air so the coolant that is in the tank makes bubble sounds and drains back in the engine. I changed the thermostat. I changed the radiator. I changed the coolant sensor under the belt tensioner. I changed the solenoid valve that opens the valve connecting the two cooling systems. I took the dpf to be cleaned . I changed the cap of the coolant tank. The weird thing is sometimes the car can drive a long trip with no problems and then sometimes it looses the coolant from the expansion tank in a small trip. I tried to see if its a gasket leak by using a gasket tester but the liquid remained blue every time. No white smoke from the exhaust. Oil cap perfect. I dont know what else to do. Any help will be very appreciated. I drive the car very smooth and not abused from day 1… basically 1000-2500rpm max. Ran out of ideas 
