Virtual library filtering random books

Hey guys,

I've been using calibre for a while now after learning what I need here and I started organizing my library using the cool virtual library thing. However, I recently noticed a strange problem and I don't know what's going on.

So I have a virtual library named standard which uses the search expression "not tag:"=Apples", not tag:"=Oranges"" (not kidding, this is really what it says), and it filters out books which do not at all conform to this (e.g. some that have "D&D, RPG" as their tags.

That's... weird. If I disable the virtual library, the book sudenly appears again.

Can anyone help me in figuring out what's going on? It'd be much appreciated!