The urls for minified files just contain a comma-delimited list of filenames. The problem is, there's no inclusion of the file's modtime or a checksum so that the filename changes when one of the source files is updated. If you use an origin-pull cdn, that means you have to manually invalidate the minified file whenever CSS or JS changes get deployed.
I wrote a small patch that inserts each file's modtime into the collection so that changing a file correctly changes the minified url, forcing a new origin-pull by CDNs. It's a quick hack, and it's not really tested outside of the scope of my own site, but hopefully it's helpful if you're having similar problems: