This is an issue that has been introduced in Update 10.0. The Big Country Labs Spoilers have the incorrect size and placement from their first appearance in Forza Horizon 5.
The 2019 Nissan 370z Nismo is one of the worst offenders with the wing being drastically shrunk down and moved up towards the rear window. As seen with Forza Horizon 5, the end plates should stick out almost as far as the rear fenders and the struts should be mounted closer to the tail lights.
The 2013 Subaru BRZ’s spoiler is also in the same boat. Like with the 370z Nismo, the BRZ’s wing was also shrunk. It was also moved forward when it should be mounted down closer to the tail lights