Both [[mb:tag/fictitious artist|fictitious artist]] and [[mb:tag/fictitious%20artist|fictitious artist]] are turned into a link to https://musicbrainz.org/tag/fictitious_artist which does not exist...
And [[mb:tag/fictitious+artist|fictitious artist]] is interpreted as a literal plus instead of a space.
https://www.mediawiki.org/wiki/Help:Magic_words#urlencode did not help as well, the mb:tag shortcut seems to use some kind of post-processing which ignores URL encoding and replaces spaces with underscores.
 
The only way I got this working is via [https://musicbrainz.org/tag/fictitious%20artist fictitious artist] (which has the downside of being treated as external link).