Did you scroll all the way down to the bottom of the tag list to check for the new tags? I couldn’t tell from your screen shot. If you haven’t already, you might want to consider installing and enabling the View script variables plug-in. This allows you to right-click on a track and display all the script variables associated with that track. I find it quite helpful here for development and debugging.
Also, the plugins you normally have enabled regarding featured artists shouldn’t have any impact on this, so you should be safe in re-enabling them.
Finally, what operating system are you using when running Picard? It shouldn’t make a difference, but I’m just curious. I have experience with both Windows and Linux, but none with Mac.