My experience with early morning fog was as following. Upon Spark reaching the layer of fog, DJI Go 4 said that the downward looking sensor experienced an error but I continued climbing. I climbed all the way above the fog (seeing that sea of clouds), then started descent, which felt a bit slow. Having Spark back, I attempted palm landing (just landing on my palm, not gesture control) but Spark hasn't initiated its usual slow landing, rather was just descending lower at high speed and I had to remove my palm quickly. Next time I tried landing, it worked.
Overall it was quite nervous and I felt a bit of unpredictability in Spark behavior. I guess it turned off the downward sensor after the error message but I'm not sure. I'll think twice flying in fog again.
Unfortunately, Airdata does not show any errors and sensors' data is all good.