Sources: 

Playground Games is actively tackling critical save file issues in
Forza Horizon 6 that have left some players restarting from scratch. The team announced, "
Our team is investigating an issue impacting some Forza Horizon 6 players where game save progress may be lost or reset," indicating a commitment to resolving these concerns.
For players on PC, verifying that their version of
Microsoft Gaming Services is up to date is essential. The recommended version is
37.114.10001.0 or higher. Users can check this by navigating to their system settings and installed apps. If their version is outdated, updating can begin through the
Microsoft Store. This is crucial, as the application facilitates the rollout of fixes for the save file concerns.
In response to the issues, players are urged to refrain from starting new play sessions or creating additional saves until their cases have been reviewed by support. The development team has mandated that users need to be online while quitting the game to mitigate data loss risks.
Furthermore,
Xbox has acknowledged the issue and is working on updates to alleviate the impact. They also recommend temporarily disabling
Quick Resume on
XBOX Series X|S for the game until the updates are fully rolled out.
Sources: 

Playground Games is addressing save file loss in Forza Horizon 6, urging PC users to verify their Microsoft Gaming Services version. Xbox acknowledges the issue impacting players’ progress, and updates are in progress to mitigate these problems as players are advised on preserving their save files.