AlbumArtist and multiple tag values

[rpatriarca]

Hi there. I am experimenting on a rather complex tagging scheme for a customer. I have devised a solution, but I face a peculiar behavior of MinimServer which is preventing me to adopt it.
To explain the issue, I refer to some sample files tagged this way:

ALBUM Le due Regine

ALBUMARTIST Callas
ALBUMARTIST Tebaldi

COMPOSER Verdi
COMPOSER Gounod
COMPOSER Bellini
COMPOSER Puccini

DATE 1952
DATE 1954

That is, a standard Album tag, two values for the AlbumArtist tag, four for the Composer tag, and two for the Date tag.
Now, if I use this tag format property string:

Album.displayFormat={$album^$date^ (^)}

I correctly get:

Le due Regine (1952, 1954)

If I then change the tag format property string to this:

Album.displayFormat={$album^$composer^ (^)}

I still correctly get

Le due Regine (Verdi, Gounod, Bellini, Puccini)

However, if I use the following string:

Album.displayFormat={$album^$albumartist^ (^)}

I only get:

Le due regine

Log files talk about conflicting tags:

restarting MinimServer[Tethys]
starting MinimServer[Tethys]
Warning: conflicting ALBUMARTIST values ignored for file Callas/Le due Regine/01 - TEBALDI - Gounod - Faust - Come vorrei saper del giovin... c'era un re.flac
first conflicting value: Callas
second conflicting value: Tebaldi
...

So my question is: why do multiple AlbumArtist tags conflict, and multiple Composer do not? Is there a way to solve this and employ multiple AlbumArtist tags successfully?

Thanks in advance,
Roberto