It seems that symfony and Encore, while allowing for source maps, are not really working very well when JS files are combined.
We have the setting enabled (see https://symfony.com/doc/4.1/frontend/encore/sourcemaps.html ) but the source map is still not working, probably because we have multiple files concatenated into one.
Brief search on StackOverflow showed this point: https://stackoverflow.com/questions/24172391/source-mapping-with-assetic-in-symfony2
Assetic, at least, has a problem where the source maps need to be in the same production directory, and there seems to be a way to attach individual source maps per file even if the files are then concatenated. This is a potential solution?
Another solution (fastest, most straight forward!) -- Remove the optimization (do not concat at all) so we can debug while we build the tool. When we go into maintenance mode on the tool (and are moving on to other projects) we can bring back the minification/concatenation.