Using id3v23 Tags causing mismatches between database and file

I need to use id3v23 tags, because windows phones 8.1 don't understand v2.4 tags.
Using id3v23 in config cause some problems: after importing tags from musicbrainz some tags are lost because they are not available in v2.3, causing a mistmatch between metadata in library and tags in file. A subsequent "beet update" will delete the missing tags in database, causing a mismatch between database and musicbrainz.

I would suggest to not compare tags that are not available in id3v23 and don't correct the database on updates.

Start by tracing how the id3v23 setting affects MusicBrainz tag import and how `beet update` compares file tags with database metadata. Done means tags unavailable in ID3v2.3 are not treated as mismatches or removed from the database during updates.

Written by the indexing model from the issue text.