+1 same issue the other night, put me off playing after a x3 length session to then be cheated by the AI...
I also feel like this isn't isolated to the MX5 Cup, likely all time attacks, as I recall during the story mode this was happening, but didn't matter as much on lower difficulty and with lesser goals/place requirements.
This seems like a weird design decision more than a bug. The AI times seem to be fixed to always be x amount faster than you on their next lap (intentionally positioning you in the top 3, but not 1st, to only be beatable once you fully upgrade the car. I've done events where I'd set a the fastest lap with a non-upgraded car after every other AI car had done their 3 laps, only to be beaten in the results screen, but then win it by a huge margin with a fully upgraded car with an only slightly faster time
