One more update from me:
finding this behaviour very inconsistent. Some artists (like mentioned in my post above with Horace Andy) only show a few appearances, though he has more in my library.
Others show zero appearances, even though they used to. (for example, Drexciya was tagged as a performer on many different aliases they released under which I had in my library)
Other albums show as appearances only under certain group members’ artist pages, but not others. This is despite all group members being tagged consistently. (For example, Mark Ernestus’ page shows “with Rhythm & Sound”, but Moritz von Oswald’s does not).
Examples too numerous to screenshot. Again, all my content is local.