An interesting quirk with Mosaic - reminiscent of the famous blurred pause button syndrome!
As a pianist I often try to download and save music that I am attemping to play. In this case it was “The Towing Path” by John Ireland. Having downloaded the tune from Qobuz I forgot about it for some time. When I looked today, some months later, I found that the song had been greyed out. Nevertheless I tried to play it - whereupon the system locked up and I could not play anything else. So, no sound from “The Towing Path” and no way to get rid of it and play something else! Deleting the Mosaic app did not work. In the end I had to shut the Bartok down and then re-start. Anybody else had trouble with ghost tracks and why do tracks become greyed out and unplayable anyway? Perhaps a case of “Up the Garden Path” rather than “Up the Towing Path”!!!
The usual reason is that the streaming service no longer has access to the copyright and it has been withdrawn. Sometimes users may not be aware as the copyright has changed hands from an owner with whom they have an agreement to another with whom they also have an agreement. However sometimes the new owner has no existing deal with Qobuz ( or A.N.Other) and the recording is excluded and has to await a new agreement being entered into. Less frequently the record label and/or publisher simply withdraw it for various reasons. These things are happening more than you may observe as repertoire ownership is pretty volatile.
I am guessing that you did not purchase a download and save it outside of Qobuz as these are basically yours for ever. So if you have repertoire that you want to be absolutely sure of having access to in the future, buy a download.Qobuz Sublime + subscription may offer a discount.