Tag loading for the release itself works for me, what doesn’t work is the artist tag fallback.
The root cause is the JSON webservice not including this data (as opposed to the XML webservice previous Picard versions used). I added an issue https://tickets.metabrainz.org/browse/PICARD-1288 for Picard. But actually this should be fixed in the web service backend, see https://tickets.metabrainz.org/browse/MBS-9400