When no release artist has been selected yet and the tracklist tab is opened once navigating back to the release tab an error is shown underneath the release artist field:

You must select an artist for the release (the background color of the artist field should be green). If you’ve entered an artist but the background remains uncolored, click the magnifying glass to match the name to a MusicBrainz artist. If the artist doesn’t exist yet, you can create it by selecting “Add a new artist” from the bottom of the search results drop-down.

Once that happens the search seems broken for the release artist field. It's no longer possible to search or paste a MB URL, although the recent items can be displayed they seem nonfunctional as well.