Chances it’s an import gone astray. I’ve run into a couple of ways this can happen. Checking your browser’s network tab to see where it is looking for stuff may give insight.
If your production root url isn’t /, (and your localhost:4200/, of course, is), absolute URLs can get you into trouble. Relative URLs with too many . .\ can also walk right past your root URL, where the “/“ wall protected you in “ember s”
If you are using .scss or .less and are importing other css, relative URLs that go outside the scope of compilation have to be relative to your assets directory, where your compiled .css will live.
I don’t know if either of these will help, but they’re both just special cases of thinking about (and perhaps reading) your minimized css where it actually lands and making sure the way you’ve expressed things will still be valid in that environment.