I have noticed what seems to be an error which occurs after entering artists via the useful collaboration feature. If you do a search in the folder pane it finds the artists individually OK, but if you browse with two fields or more, e.g. Artist + Composer, the composer linked to items which have been entered using artists in collaboration do not appear in the tree folders on the left but are there on the right.
I hope the attached screenshot illustrates this: On the left Christoph Eschenbach only appears to be in my collection playing Schumann (where he plays alone). On the right it can be seen that he also plays Schubert (duets with Justus Frantz), yet Schubert does not appear on the left.
I have used collaboration a lot and the same thing happens in all cases.