Why has some of my albums become unavailable in Roon (after having loaded them and listened to them many times) but are still available directly from Qobuz?
It is because Qobuz has replaced the version of the album that you added to your Roon library with a newer version. On the album page, you can click/touch the Versions tab to add the new version.
See here for a way to track down these albums so that you can update your library with new versions: