I have updated an entire album with feat. Artists in Picard. I could see the changes in the tag inspector, but nothing, up to restarting my whole computer, would make them appear in the main panel (first image).
Finally, I dug around a little more in the tag inspector and discovered that there was a Performer (TMCL) tag attached to the file that was not showing up in the Multiple Artist splitter (see below). Picard did tag the performer tag without a role, so that was correct. Deleting that tag solved the display artist problem. However, now when I go into the tag inspector, there are still TMCL and TIPL tags shown - they're just blank. If I delete them, they reappear as soon as I reload the tag inspector.