I am not able to reproduce the issue yet. It seems to be something that happens only on rare occasions; Also, I am not a web developer by trade, but do firmware stuff, so the chances of me reproducing this issue is far less, since using CSS doesn't often crop up in my day to day coding activities; only in hobby projects do I use CSS. I am not the lead developer on this either; I am only someone who uses this plugin. I kept thinking Ask would be looking into it, but he is probably busy; all development has been quiet on his end.
If I could figure out what combination of events causes this issue, I could fix it, but like I said, I cannot seem to make it do what you have described. Not on Linux, Windows, or Mac. Since this is not my plugin, finding the issue has been very low priority for me since all is working on my end.
It is possible I could put a safe guard on all instances where the path is assembled to ensure path integrity. This may be the way to go because it doesn't require me to determine exactly where the issue is, but bandage it if it is detected. Ideally though, it would be nice to discover why the path pieces are not as expected on your system, and patch this issue properly.