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Just to make sure… you know that there are at least two different 404 pages, one that the webserver returns when the URL is not defined in the webserver config and second, the one that is returned by Django, when the webserver handed off the request to Django but the urls.py does not define a pattern matching the URL?
In case you are aware of that and the 404 pages with and without static content displayed in your browser are both served by Django, I would suggest to look at the requests the browser sends to get the static content. You can do that in the debug console of Chrome, Safari, Firefox. There you’d be able to see what’s different with the requests (likely the URLs) so that the webserver is returning the static content for the one but not for the other case.