White smoke from exhaust

[Efterlyst]

RS6 C7 pre facelift
There is huge cloud of white smoke after driving for a while, no loss of power and only when putting a lot of gas.
I ordered cylinder top gasket test kit and put it in the coolant reservoir and the fluid was blue all the time. There is however small bubbles in that reservoir.

what could be causing this?
however I google I always get top lock gasket answere. Could it actually be leaking but the car has to be super hot?