I 've countered this same thing , a few years back.
And found out that it's not a glitch , but just the game trying to match you with cars with similar performance.
For example : you select a
Race car (about 600pp) from favourites and you get a AI grid of
Race cars (~600pp)
Select a
road car (~550pp) and you'll get AI grid of
550pp road cars.
I don't remember the exact pp#'s where the AI cars change and on road cars end up being just Citroën GT Concepts.
There is a way to fool the grid matching by simply removing power adding parts / power limit / ballast a specific car in GT Mode before taking the same car to Arcade mode races.
So , reduce the pp to lower level to fetch a grid of easyer AI cars.
And change the pp back to where is was before limiting & stripping the car in GT Mode when the race & Ai grid has loaded.
Try a Ginetta G4 '64 maxed out at Arcade mode and you'll get a grid of Veyrons , F1's and Speed 12's IIRC.
