THE FINALS has a repeating issue where major updates will break it in some way, with the latest update causing the game on Desktop Linux / Steam Deck to freeze a few minutes into a match.
After numerous reports flowed in to the Proton GitHub page for it, Valve managed to find the cause of the problem and have released an update to Proton Hotfix to solve the freezing. Valve have also now set Proton Hotfix as the default for it, where it was previously set to Proton Experimental.
If you have manually set THE FINALS to a different Proton version, you should untick the compatibility box to let it automatically go to Proton Hotfix. More on how to change Proton versions in my guide. If you don't have Proton Hotfix downloaded, it will automatically get downloaded on Steam when you go to launch THE FINALS. As long as you don't have a different version of Proton manually set on it.
Testing myself with Proton Hotfix today and a full match worked just fine.
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