Scrollwheel binding doesn't work properly

I want to bind ‘scroll mousewheel up’ to jump. This works properly. I want to bind ‘scroll mousewheel down’ to switch weapons.

However, as soon as I save the settings—even after restarting the game—the weapon switch remains bound to ‘scroll mousewheel up’. The only way to prevent this is to completely unbind the weapon switch function from any key. But then I can no longer switch weapons.

It seems like the game automatically reverts to the old scroll wheel setting for weapon switching. Hardcoded?