[schubeda]

Actually Picard does not necessarily separate multiple album artists (or artists) with a semicolon, it just displays tags that have multiple values as a single line using semicolons on its UI. The actual metadata values may not have any semicolons at all, it all depends on the tag format. Formats like ID3 v2.4 (mp3 files) or Vorbis comments (FLAC files) support multi-value tags, and Kodi has no problem in reading and storing the separate values. However older ID3 v2.3 format is single frame, and the standard is to use " / " as a separator, but it sounds like that isn't what you have.

When you tag using Picard it provides:
a single string value for  ALBUMARTIST = "Composer: Conductor; Orchestra; Soloist"
but multiple values for ALBUMARTISTS = "Composer",  "Conductor", "Orchestra", "Soloist"
which is displays on the screen as a line of values separated by semicolons.

Kodi reads both lots of data, shows ALBUMARTIST string as the album artist display, but lists the individual artists (from ALBUMARTISTS) in the artist node.

Assuming that the music files that you manually tag are using the same format, then it sounds like the tagging tool is either not providing an ALBUMARTIST value or is converting the separate ALBUMARTISTS values into ALBUMARTIST "Composer / Conductor / Orchestra / Soloist". What tagging tool are you using, and what format of tagging?

If you provide an example of a manually tagged classical music file then I can tell you why you are seeing what you are seeing.

I'm posting (partial) outputs of `metaflac --list filename.flac`
First file (tagged by Picard):
    comment[0]: MUSICBRAINZ_RELEASEGROUPID=dd7c3b33-d6af-4b47-b84e-a2819628f2ae
    comment[1]: ORIGINALDATE=2016-04-29
    comment[2]: ORIGINALYEAR=2016
    comment[3]: RELEASETYPE=album
    comment[4]: MUSICBRAINZ_ALBUMID=7b689f15-f25d-4a78-91a9-a0719626572e
    comment[5]: ASIN=B01DC8N3XW
    comment[6]: MUSICBRAINZ_ALBUMARTISTID=1f9df192-a621-4f54-8850-2c5373b7eac9
    comment[7]: MUSICBRAINZ_ALBUMARTISTID=dea28aa9-1086-4ffa-8739-0ccc759de1ce
    comment[8]: MUSICBRAINZ_ALBUMARTISTID=80c3bce9-0b54-409c-aa10-3b0c7dbbc597
    comment[9]: ALBUMARTIST=Beethoven; Berliner Philharmoniker, Sir Simon Rattle
    comment[10]: ALBUMARTISTSORT=Beethoven, Ludwig van; Berliner Philharmoniker, Rattle, Simon
    comment[11]: LABEL=Berliner Philharmoniker Recordings
    comment[12]: CATALOGNUMBER=BPHR 160091
    comment[13]: DATE=2016
    comment[14]: ALBUM=Symphonien 1-9
    comment[15]: BARCODE=4260306180912
    comment[16]: RELEASESTATUS=official
    comment[17]: TOTALDISCS=5
    comment[18]: MEDIA=CD
    comment[19]: DISCNUMBER=1
    comment[20]: TOTALTRACKS=8
    comment[21]: DISCSUBTITLE=Symphonien Nr. 1 & 3
    comment[22]: MUSICBRAINZ_TRACKID=034d6e1d-a45f-4587-b2bf-484fc4b15faa
    comment[23]: MUSICBRAINZ_ARTISTID=1f9df192-a621-4f54-8850-2c5373b7eac9
    comment[24]: ARTIST=Ludwig van Beethoven
    comment[25]: ARTISTSORT=Beethoven, Ludwig van
    comment[26]: ARTISTS=Ludwig van Beethoven
    comment[27]: TITLE=Symphony no. 1 in C major, op. 21: I. Adagio molto - Allegro con brio
    comment[28]: CONDUCTOR=Simon Rattle
    comment[29]: PERFORMER=Berliner Philharmoniker (orchestra)
    comment[30]: PRODUCER=Christoph Franke
    comment[31]: ENGINEER=René Möller
    comment[32]: MUSICBRAINZ_WORKID=80297632-7cf5-3dae-b36e-230c87ef8c30
    comment[33]: LANGUAGE=zxx
    comment[34]: WORK=Symphony no. 1 in C major, op. 21: I. Adagio molto - Allegro con brio
    comment[35]: COMPOSER=Ludwig van Beethoven
    comment[36]: COMPOSERSORT=Beethoven, Ludwig van
    comment[37]: MUSICBRAINZ_RELEASETRACKID=af706a7f-ce80-4892-8475-d459abd190c9
    comment[38]: TRACKNUMBER=1
    comment[39]: ALBUMARTISTS=Beethoven
    comment[40]: ALBUMARTISTS=Berliner Philharmoniker
    comment[41]: ALBUMARTISTS=Sir Simon Rattle
    comment[42]: ACOUSTID_ID=7e12735d-71db-463f-bafa-1f141168aa0f
    comment[43]: GENRE=Classical
    comment[44]: TRACKTOTAL=8
    comment[45]: DISCTOTAL=5

Second file (tagged by hand using Puddletag, some data might be from freedb, some data by the ripper)
    comment[0]: album=Pianoforte 8
    comment[1]: artist=Ludwig van Beethoven
    comment[2]: musicbrainz_discid=k7HZ2KxQFGRK.MVc7NzQMusd6PA-
    comment[3]: title=Concerto no. 3 in C minor: Allegro con Brio
    comment[4]: tracknumber=1
    comment[5]: genre=Classical
    comment[6]: freedb_disc_id=240cf804
    comment[7]: freedb_category=classical
    comment[8]: date=1994
    comment[9]: albumartist=Beethoven; Wilhelm Backhaus, Wiener Philharmoniker, Hans Schmidt-Isserstedt, Lili Kraus, Amsterdam Philharmonic Orchestra and Chorus, Gianfranco Rivoli

In Kodi, when I list all albums, in the first case it shows `Beethoven; ...`, whereas in the second case, it shows `Beethoven / ...`