So this issue is bizarre and I hope I can make an example that does this.

I successful got sourceMaps to work with stylus-loader with
{ test: /\.styl$/, loader: 'style!css?sourceMap!stylus' }

However when ?sourceMap is enabled all our background url('whatever/sprite.png') no longer work. When the ?sourceMap is removed all is well.

I will make an example repo when I get the chance. Also let me know if this should be an issue on stylus-loader instead.

As for more info the css produced are identical except that sourcemap css is redirected via @import some url. The computed CSS reported by Chrome Debugger is identical, but using (Chrome Extension) CSSViewer reports different processed CSS. Which shows the missing sprite.

Also this problem exists in Firefox as well so I do not think it is a chrome issue.