I did what you suggested, and, indeed, they are grouped in the same album and are not affected by the sorting or grouping of the view. The conditionals only pertain to the <Grouping> tag (which is a string field), and, as I metioned earlier, have the expected values.
Digging a little more, I created a new virtual tag <LV> that simply translate the numeric value in <¿Lista, Vinil-r?> (1, 2, 3) into a string value ("A", "B", "C"), and this new tag works every time. I tried to change the configuration of <¿Lista, Vinil-r?> from numeric to string, but MB doesn't allow me to.
Then I changed the definition of <¿Lista, Vinil-r?> to be "A", "B", "C" from start (and make sure that it were considered as string in its configuration) and it was the same as always: It worked just for some <Album Artist>s (the same as before). So I changed the name of <¿Lista, Vinil-r?> to <ListaVinil>, and it was the same, erratic behaviour. I even changed the corresponding virtual tag position (from virtualtag8 to virtualtag14) to no avail.
I think this may be a bug. For what is worth, this is the definition of the virtual tag in question (formerly, "B" was 2; "A", 1; and "a", 0).
$If($Left(<Grouping>,3)="[L]",B,$If($Left(<Grouping>,3)="[C]",B,$If($Left(<Disc Subtitle>,1)="_",A,a)))