Like Frankz, I cannot see why this is not working for you - the same search is working for me.
A couple of things you might like to try:
1. Create an auto-playlist with the same rule (<Artists: Guest> has a value) to see if that gives you any results.
and/or
2. Create a new tab - select Music node - set view to Tracks. Then Set Displayed Fields adding <Artists: Guest> (and maybe also <Artists: Artist>).
Neither of these actions will explain why your custom search is not working but may show that you have things set up correctly (which you appear to have) and may offer you a way to proceed.