SourceMap not working

When launching a frontend (e.g. in the custom_greeting tutorial) with the url, the console reports

DevTools failed to load SourceMap: Could not load content for HTTP error: status code 404, net::ERR_HTTP_RESPONSE_CODE_FAILURE

How can I get my SourceMap to load? Not sure how it would ever successfully load the SourceMap at that url; how would it know which canister’s SourceMap to load?

As in the tutorial, I have devtool: "source-map" specified in webpack.config.js.

edit: this seems like a question for StackOverflow. Are we encouraged to ask this kind of question here or there?