The issue is DirectX emulation. Source physics are suuuper broken in wine wrappers and other Linux compatibility layers. Proton is developed by some of the folks that invented DirectX and properly interprets the API calls nearly natively at runtime.

Source: I tried getting multiple source games working in old Linux wrappers pre-steamdeck. One explosion or fast-velocity physics object and the entire game crashes. It just can’t get the timing right and freaks out.