I noticed the referenced condensation recently, as well. In short, my '18 CGTS was outside during a decent rain one evening. Though the car was hastily garaged upon my return home, I happened to notice (upon return from a trip five days later!), that almost the entire left rear tail light was fogged up (after having not been driven for days).
Fortunately, I had a service appointment already setup that week, and with the photo that I had taken, the dealer replaced the light gratis. When I arrived back home after the service appointment, upon examination there was actually still a bit of fogging (~20%) on the same light, though this has since been ameliorated.
In any case, the service manager attempted to elucidate something about how the LED light / enclosure may not have been properly engineered or some such thing (umm, I bought a Porsche...), and to some extent the problem is considered 'normal' and 'not an issue' for the most part.
All this being said, I wonder where the H2O is hanging out (for days while parked) -- potential root cause?