Could it be that there is a miscalculation somewhere? I've noticed that the AI gets extremely faster from lap 10 onwards in rain races. They should actually slow down just like we do. But they keep getting faster and faster. In F1 World in the event invitations the same thing happens only in reverse. If my car gets above 500, I get slower and slower, the higher my level goes, the slower I get. I am now over 900 and am 5 to 7 seconds slower than the AI and other players. I even get lapped in 18 lap races. Back to the rain race. I've worked on the tires and had to increase the y value under weather grip from 0.99 to an incredible 1.99 to be able to keep up with the AI at all, which is really tough in my eyes.
I was able to pull out a lead of around 20 seconds within 10 laps. The AI strength was set to 100 percent and the race in Australia lasted 58 laps. From lap 10 onwards, the AI became so fast that it was right behind me within 4 laps. I drove a 1.24, the AI a 1.20! It couldn't be down to the grip as I had pulled out a lead. My ERS was shortened for hotlap from lap 8. Before that I had hotlap for 4 laps, from lap 8 only for 1 lap. It can't have been ERS alone, I don't think Hotlap makes up 4 seconds per lap. Either this is a calculation error or the AI gets even more engine power later on. I just find it a bit strange as in rain races and in F1 World Event the problem is very similar, only in reverse. I did the tire mod to see where the problem is. You can test it yourself. Maybe it will help you with troubleshooting. Here is the link to the mod.
https://www.overtake.gg/downloads/tire-mod-controller.77874/