Perhaps the engine was not designed to just burn fuel at idle for extended periods to simply warm it up during a lay up
Use it throughout the year by driving as it was intended even if this means waiting a month in my case for a gap in weather/salted roads given the awful winter we are having this year.
If the problem exists when driving the car regularly then consider the PCV kit. Gen2 B58 engines use hybrid direct and manifold injection so may not be as vulnerable to carbon build up on valve stems as earlier pure direct ones.
