Reproducible in Opera and Safari (both OS X)

e.g. http://test.musicbrainz.org/release/f3502c46-7762-47ae-be1d-d6da82e6fdac/edit

Go to the tracklist page, click the end of one of the lines, backspace up to the hyphen, then retype the space and the artist name. The preview doesn't update and if you try to continue, it says that the artist is required.