[DaveBlake]

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.