This issue has persisted since Forza Motorsport 4, where this car debuted as paid DLC, but it is likely to be derived from the Motorsport 2 DLC Audi S5. The Audi RS5 8T has got one of the most inaccurate car models out of any car that debuted in Motorsport 4. From issues with the side line, missing and low quality details, to abhorrent body ends, it is somehow as bad as some Motorsport 1 models.
I believe these comparison gifs are self-explanatory, even if not lined up 100% correctly. Every single bit of the front end of this car is inaccurate, but the most egregious inaccuracy must be the front bumper - the side intakes are SIGNIFICANTLY too large, but overall the front bumper feels not curved enough. The headlights are very low quality and lack details.
Rear end differences here are likely to be caused by perspective, but the lights are definitely extremely low quality. The rear bumper feels too small as well. The stock Rotor wheels are lower quality than on all other cars that come stock with them (RS4 B8, RS3 8P) and lack separate paint groups.
Overall, this model needs to be replaced with a new and accurate scan, one that can also accommodate other RS5 trims. It is a good thing that this car did not appear in FM2023, but it also shouldn’t return in any next Forza game without getting remodeled.