Help needed to track down inconsistent metadata setting

[simbun]

I don't have the Naim app nor any Opera so I'm going to answer in relation to what I know, so apologies if that isn't relevant.
When you click on one of those 5 objects (which I'm assuming is an Album view), I would expect the next level to be a track listing, unless it's not a complete album, so are you sure the other 4 Mozart items represent full albums?

Another way you might be able to debug this yourself is via MinimWatch; in the log window set the logging level to Debug and see what's sent to the control point when you select either of those items (set it to Debug just before you click on any of the 5 items, and change it back before you click on anything else). In the resulting log search for the string 'container' e.g. 'browsing children of container com.minimserver.Server$AlbumContainer@', shows MinimServer is classifying this object as an Album.

EDIT: Setting MinimWatch > Advanced > showExtras to true may also give you some more context in the control point, but if I had to guess I'd say that Idomeneo is the full album whereas the others aren't.