I've checked the github issue queue, and this forum, and hadn't been able to find anything on this.
Looking through the MDN Content-Disposition Docs (I would link, but was not allowed)...
Content-Disposition: attachment; filename="filename.jpg"
would set a filename different from the url when a user in a browser clicks 'save as'. I would expect
--auto-filename to respect that header if it's been set, it sounds like that setting should do that.
Instead, as per the copyurl docs, it does:
-a, --auto-filename Get the file name from the URL and use it for destination file path
Any reason rclone could not get a little cleverer here and respect the filename directive, if set?