[User 421137]

I thought Vorbis comment is not case sensitive. Does anyone know more precisely where that's not the case for Kodi?

You are right. Vorbis comment is specified as case-insensitive.
Maybe it is working well for Vorbis comment but I don't know it in detail. I don't how it is implemented in detail in Kodi and in the used library TagLib (or in other libs used for other products).
Maybe you run into a problem later if you use a different tool (and don't clean existing tags). E.g. you might get two tags, one in lower case, one in upper case.
Basically all tags in Vorbis comment can exists multiple times. Only the systematic of the tagging software (e.g. Picard) determines if a tag is a single-value entity (i.e. tag exists just once) or a multiple-values entity (i.e. tag exists multiple times in case of Vorbis comment).

Code:

C:\_tmp\Music>C:\Portable\flac-1.3.2-win\win64\metaflac.exe --set-tag=ALBUM="Best of All" silence.flac
C:\_tmp\Music>C:\Portable\flac-1.3.2-win\win64\metaflac.exe --set-tag=album="Best of All" silence.flac


C:\_tmp\Music>C:\Portable\flac-1.3.2-win\win64\metaflac.exe --list silence.flac
METADATA block #0
type: 0 (STREAMINFO)
is last: false
length: 34
minimum blocksize: 4096 samples
maximum blocksize: 4096 samples
minimum framesize: 118 bytes
maximum framesize: 1266 bytes
sample_rate: 44100 Hz
channels: 1
bits-per-sample: 16
total samples: 4410
MD5 signature: 49521a666d6ecf9ccbe0bd35c84c76ce
METADATA block #1
type: 4 (VORBIS_COMMENT)
is last: true
length: 82
vendor string: reference libFLAC 1.3.1 20141125
comments: 2
comment[0]: ALBUM=Best of All
comment[1]: album=Best of All


C:\_tmp\Music>C:\Portable\flac-1.3.2-win\win64\metaflac.exe --show-tag=Album silence.flac
ALBUM=Best of All
album=Best of All

 

This applies to ALBUMARTSTS, but it's ok to have a semicolon in ALBUMARTIST, correct?

Yes, that is basically correct.
"ALBUMARTISTS" or "albumartists" (it's a user-defined tag for Picard, typically set with a script) has multiple values. Based on the used metadata container there are several tags (like for Vorbis) or the values are concatenated into a single entity (e.g. ID3 v2.3).
"ALBUMARTIST" is a single-value entity for MusicBrainz/Picard. You can use semicolons or whatever you want; or set it based on the Style Guides (e.g. of MusicBrainz); or just set it how you want to see the album-artist(s) displayed in Kodi.
In case you have a correct "ALBUMARTISTS"-tag in your files (and Kodi can read it correctly) it's somehow irrelevant what separator you use for the tag "ALBUMARTIST". However, it is the field shown as artist(s) on album-level.
Just in case you don't have a correct "albumartists"-tag (or Kodi can not read it) and it is a multiple-artists-release (multiple MusicBrainz Release Artist ID values) Kodi tries to find the individual artists in the tag "ALBUMARTST" (using the
defined artistseparators).

N.b.: "albumartists" is a user-defined tag for Picard and it is written in different style, depending on the used metadata container, into the files, e.g. in upper case for Vorbis comment and in lower case (i.e. in the style it was named) for APEv2 (.m4a files). See thread M4A files with ALBUMARTISTS tag MBID Shown as Artist Name.


P.S.: The tag mapping for Picard is well documented but sadly without info if single or multiple values.