Also noticed this today. Although i have to say i got quite lucky and didn't get rain for the first 2/3th of the race. I did 2 stints on RH and both i had to pit at the end of the 8th lap. So on lap 8 and lap 16. Tires were not completely gone, but i started to really notice the lack of grip. I cannot compare to before because i simply just don't know how many laps i did pre-update 1.20. I'm playing with controller, TC 0 and was driving the Audi R8 on FM6.I have noticed something in my Spa 1 hour race today. A number of AI cars start the race on RMs, then pit after about 6 laps. They still have one third fuel and there was no rain. So their medium tires have worn apparently.
PD does not seem to have realised the speed of tire wear which their changes have caused. Otherwise they would have changed the AI tire strategies, which are now invalid. So my guess is they screwed up the change. I hope they will make some adjustments next update