Like the title says, Drivatars in Trials are using stock PI cars. This was introduced with the update for named Drivatars and was first seen during the Trucks Trial (Series 8 Week 1 Summer).
Players assumed that this was because the Trucks Trial might have been too difficult but it is clear by now that it is applying to all Trials.
If this is a bug please fix it by next update.
If it is intentional please revert this change. There is zero challenge in Trials now. I understand that some players didn’t know better and would take stock cars into Trials but these players aren’t going to magically start winning against stock cars because they already lack the basic knowledge and skill!