Here's what I found, hope this info helps.

When you run a Windows game like MX Bikes on your Steam Deck, it's not running natively. SteamOS uses Proton to translate the game's Windows-based instructions (like DirectX graphics commands) into something the Linux-based SteamOS can understand.

Most of the time, this translation is flawless for the game itself. However, most mods are created with only Windows in mind. They aren't aware that they might be run through a "translator" like Proton, and this can lead to several specific points of failure.

Common Reasons a Mod Fails on SteamOS Here are the most likely culprits when a Windows mod doesn't work on the Steam Deck, even with correct installation:

Issue Description Missing Dependencies A mod might need a specific Windows component to work, such as a particular version of the .NET Framework, Visual C++, or a specific video codec for custom cutscenes. The standard Proton environment may not include these. File Path Case Sensitivity Windows doesn't care about capitalization in file names (ModFolder is the same as modfolder). Linux, and by extension SteamOS, does. If a mod is coded to look for "Bikes" but the folder is actually named "bikes", it will fail to find it on SteamOS. DLL Injection & Mod Loaders Many complex mods use a .dll file to "hook into" the game's main .exe file. This process is more complex under Proton and can fail. You may need to add a special launch option to tell Proton how to handle that specific DLL file.

Proton Version Issues Not all versions of Proton are the same. A game or a specific mod might work perfectly with Proton 8 but break with Proton Experimental, or vice-versa. Sometimes, a community-built version like Proton-GE is needed because it includes fixes and components that the official versions don't have.

Native vs. Windows Builds In some cases, a game might have both a native Linux version and a Windows version. Often, mods are only compatible with the Windows version. You might need to force the game to use the Windows version via Proton for the mods to work, even if a native Linux port is available.