Hello,
I understand but I used the Linux export and even repeated it but the issue remains and Steam support say everything is correctly set up.
Sounds like the game is being started with the Proton compatibility layer. In that case, Steam downloads/installs the Windows build you've uploaded.
So it's possible that you haven't properly configured the Linux build on Steamworks. Or that the player has manually chosen the Proton compatibility layer in the game's properties.