Forza Motorsport's launch has been plagued with issues, and developer Turn 10 Studios will likely need to work with Valve to get the game compatible with the Steam Deck and its Proton interpreter.
An update to Forza Horizon 5 has also seen that game suddenly stop working on the Deck, with several users writing that both games crash before reaching the splash screen.
Valve coder Pierre-Loup Griffais has been notified of the issue, and is "looking into it." Despite this, the games still show a 'Steam Deck playable' status on their store pages.
Similar issues have been reported by Linux users that have attempted to play the racing games via Proton, a software developed by Valve. Proton is a translation layer designed to allow Linux users to play games that are otherwise only compatible with Windows, and the Deck uses it to run the large majority of its games.
Despite recently falling from the top 10 global best sellers, the Steam Deck is still massively popular. Many use it as their primary way to play games, so it's a shame that this new release has neglected it. Still, the game had reportedly been experiencing several bugs and even save file corruptions in the days leading up to launch, so it will hopefully be in a more stable state by the time Deck and Linux users get to play.
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Turn 10 Studios is owned by Microsoft, so the Forza games' tendency to target Windows and Xbox above all else makes sense. Still, there is hope among fans that that the studio will work together with Valve to get the games working for everybody who wants to play.