Tags in MusicBrainz are generally comma-separated, and most of our code does not allow adding (or editing) tags with commas in them. The API POST method though will happily take this:

{{ <user-tag-list>
<user-tag vote="withdraw"><name>wait, really</name></user-tag>
</user-tag-list>}}

And submit a "wait, really" tag with a comma. This later causes issues when trying to edit it through the website (MBS-8697).

The API should probably block tags with commas in them and return something like "Commas are not allowed in tags. To submit multiple tags, send multiple <user-tag> elements".