So I think i've seen someone else have a similar problem, but there's now a glitch where the AI won't go onto wet tyres when the track becomes wet due to rain, they will pit over and over again but will keep going onto dry tyres. This happened to me in a career mode race, i'm not sure if it happens outside career mode but you really need to look into this codemasters. Completely ruined a really great race I was having. I haven't been on the game over christmas but it didnt happen before then so i'm not sure if this is due to one of the updates you've recently brought in which has caused an error in the game.
Please do something about this, despite some bugs overall this game is really good to play but this has changed it completely