@Redwing
I've followed all your steps and everything happened as expected. It's true that I did send you the captions after I deleted all the tags: you were asking me to try things and, at the same time, I kept having ideas that I wanted to put in practice. I guess I should have reported my problem once I had exhausted all my ideas, though when wrote my first post, I was completely lost. I apologize.
Having said that, I'm almost completely sure that there was no <ALBUM ARTIST> in that album because I don't like this tag and I make sure I delete it sistematically. It could have been the case that this tag was hidden if there's certainly such a thing as a hidden tag (I came up with this idea of hidden tags because I had once a problem in the Winamp library and somehow I fixed it by removing all the tags using that feature in MP3tag).
There's a big difference, though, between today's scenario and yesterday's. Today I could see the other tracks somewhere else, in a different group: I could, for example, search for track 27, which has a distinctive name. However, yesterday it wasn't the case that these four tracks were somewhere else: they just disappeared from the library as far as I could see.
If you're still interested, I could ask my brother to send me the album again, with the tags intact, as I keep a copy of my library in a hard drive in his flat.
Thanks!
@Steven
I'll take into account what you said about <F5>. What I did before, when I rescanned an album, was to go somewhere else in the library and move back in order to refresh the information.