I believe that WMP primarily sorts on 'album artist', not artist. If you import an album that doesn't have the 'album artist' tag set, then it could do the following. If all songs have the same 'artist' tag, then it will assign that value to 'album artist'. If any 'artist' values are different, then it assigns 'various' to 'album artist'.
If you import an album with different 'album artist' values for different songs, then it will create in its library multiple Albums, one for each different value.
WMP seems to make reasonable decisions on how to sort things, but the I've got to go out of my way to make it sane when presented with various artists albums, such as sound tracks. Then I set 'album artist' to be the name of the movie, or if a compilation, however I want it. The nice thing is that I have my entire "Hits of the 80s" collection imported with that as the 'album artist', so I can select the whole collection that way.
My suggestion is to edit the album from within WMP, or delete the album and then re-import it after editing with Mp3tag.