i had this problem as well and ended up changing the ->Stylesheet call in the theme to the actual url (getThemePath etc).

the caching of css makes little sense to me in most scenarios sense it will still result in an extra call to the server however you do it. and since the caching isn't working as expected (even when every caching setting is "disabled") it was a no brainer to not take advantage of it.

also, the caching for the Stylesheet method seems to assume you have your css in the root dir of your package which is usually not the case which makes it unusable anyway.