This is in Amarok 1.4.9.1 using KDE 3.5.9 in the Context > Music tab.

I noticed this with artist Alison Krauss. I have some tracks by her on movie sountracks, which are listed, whether or not she is listed as singing with her band. And I have a solo album, which is also listed properly. However, the album with Robert Plant isn't listed, I think because Robert Plant's name is first. I consider this a bug.

A search or a sort of the collection finds that album and all the individual tracks properly.