<metaTag name="source"> empty when downloading from musescore.com

As far as I understand the <metaTag name="source"> element in a MuseScore file is used to associate a downloaded file with the file hosted on musescore.com. This worked a few times for me. I could download the file, open it in MuseScore, go to "File => Save Online", and I get the checkbox "Update the existing score".

But most of the time when I download one of my files from musescore.com, there is no number in the <metaTag name="source"> element. What can be the reason for that? As this file is generated server side, I don't expect the browser will have any influence, but I nevertheless tried both Firefox and Chromium with all addons disabled - unfortunately without success. I can add the number manually, but that's of course more labor intensive than updating it on musescore.com by clicking "Edit this score => Replace".

Any help is appreciated.