Thanks for the hints.
It does look as if most (if not all) of the albums affected are compilations.
It took me a while to track down a decent metadata editor but I’ve now had a look at one affected album and there was no Album Artist set (there was a “compilation” flag set).
Setting the Album Artist to “Various Artists” and refreshing metadata has no effect (other than to group the entries together when sorted by artist).
Removing the data from the server, rescanning, emptying the recycle bin, cleaning bundles, putting the files back and rescanning as described in the “Plex Dance” article also has no effect.
There has been no change to the metadata between having everything working as expected and upgrading the server and moving the data onto a new share so I still don’t see what might have caused the change in display.
