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Copy "d3d12.dll" to the game's main folder, where the executable file is located.

Example:
...\Forza Horizon 6\d3d12.dll
The correct folder is the same one where several .dllgame files and the main executable are located.

Open the game normally.

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The fix also creates a log file in the game folder called "ForzaFix_RX580.log". 
If this file appears, it means the DLL has been loaded.

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How to remove the fix:
just delete "d3d12.dll" from your game folder.
The game is back to using the normal Windows DirectX.

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Source:
https://github.com/Megadroidgames/Forza-Horizon-6-RX-580-FH201-FH205-Fix
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