IIRC it was that the new CSS file wasn't being tagged properly when being created...if I went in and put a space somewhere in the css file it would tag it as "updated" or somesuch and then it would save the file properly. Otherwise it had a tendency to clear the entire sheet...leaving a blank css file.
let me see if I can find the thread where we talked about it. I think Kevin was the one to find the problem and fixed it for 8.7.
Edit:
Here it is from
this thread