Why isn't the geo-specific fallback not working?

Bottom line: the parameter desktop_url should not exist on the Quick Link

When creating a new link: do not set any desktop_url manually when making the link, just keep it as default. Default means that we we will read from the app table settings. If you set it manually, you are explicitly defining it and this may cause issues.

If the link has already been created and the redirect is not working: to make the current link work, you can reset the desktop_url to default redirect and test again.