Music on iPhone/iPad grouped bizarrely despite being fine on iTunes
Found it!
There is an additional field in the data: “Work”.  It doesn’t appear in the “Edit info” box, but you can make it appear as a column in playlists and the like.
The misbehaving tracks had had “Work” values set; I deleted them, synced it across, and then they were showing up as expected.
Note that editing this field doesn’t seem to qualify as an “edit” for the sync to pick up automatically, so I had to modify something else to make it re-copy those files.  (I added a space to the end of the song name, synced, then removed the space and synced again.