I also experienced this rain-issue on my first private lounge after downloading the track.
But I did definitely felt an serious degradation of the grip.!
At first, I didn't feel any lose of the grip, but after getting used to the track and starting to do some adjustments on the setup of the car, about 45 minutes into the session, I seriously experienced that the grip was affected.
At first I thought it was just my setup-tweaks starting to act weird, but after some more desperate adjustments just trying to stop the wheel-spin (in a cappuccinno..!! 👍) that has started to occur in 2. and even 3. gear on straight-road acceleration, I finally realized it was due to the track getting more and more wet...
Got safe back to the pit, changed to full rain-tires, and I was now back on my initial times..
So the weather definitely has impact, even if it is not obvious from the start...!