There is a post somewhere in FH5 Discussion about this but it’s buried. As far as I know there hasn’t been a post here in the Troubleshooting Hub about it. This issue was present in Forza Horizon 4 as well
The 2019 Raesr Tachyon Speed cannot be equipped with race-tier anti-roll bar upgrades, nor are the anti-roll bars adjustable when the car is stock.
Being able to adjust anti-roll bars is critical for fine-tuning the balance of the car during cornering. The spring rates and downforce balance can be adjusted to compensate, but it’s preferred to adjust oversteer/understeer with anti-roll bars as opposed to changing spring rates.