sass conflict leading to an unusable main.css locally

When I generate the site, the output includes:

          Conflict: The following destination is shared by multiple files.
                    The written file may end up with unexpected contents.
                    /home/aengelen/dev/reproducible-website/_site/assets/styles/main.css
                     - assets/styles/main.scss
                     - assets/styles/main.css.map

... and sure enough, ./_site/assets/styles/main.css contains something that does not look like CSS (I guess it's a sourcemap).

Upgrading jekyll and related components doesn't seem to help. Adding sourcemap: never to the sass section of _config.yml works as a workaround, but I guess we should report/fix this somewhere upstream instead - as it doesn't appear to affect the 'real' website (yet?).