Default Application incorrectly listed as User-set

Steps to reproduce:

1. Open xfce4-mime-settings
2. Select a mime type that uses the system default, e.g., text/html set to Firefox
3. Edit ~/.config/mimeapps.list to add a line assigning this mime type to an invalid/missing desktop file (e.g., text/html=this-app-is-not-installed.desktop)
4. Close and reopen xfce4-mime-settings
5. The mime type still (correctly) shows the system default, but now indicates that this is user-set