Sometimes individual tracks disappear from albums on streaming although the album as such remains available. Probably rights issues. May be just that.
(Such tracks are sometimes still in the track list but marked unavailable in red. Sometimes they are simply missing from the list and not easy to spot, in particular if track numbers are not visible)
Ah, so you can’t remove them at all? I thought you just get the message about missing files. If you can’t remove them, that’s weird I guess. Would be interesting if tracks are missing/unavailable if you look at the album in Qobuz