<Artist: Guest> is not showing up among the selectable fields within the "Replace with:" box (red arrow) whereas it is selectable in the "Search for:" box.
In Musicbrainz, featuring artists were included in the title in the past (as displayed on the CD tray for example). However, they recently changed their policy to move that info in <Artist>, as seen in the following (<Artist> - <Title> format):
Before Musicbrainz change: Artist - Title (feat. Guest Artist)
After Musicbrainz change: Artist feat. Guest Artist - Title
The change is necessary for Musicbrainz's db purposes but it leads to sorting issues if one sorts by <Artist>. I've been trying to do the following with "Foo feat. Bar - Title"
Example (<Artist> - <Title>): Foo feat. Bar - TitleFrom Musicbrainz
<Artist> = Foo feat. Bar
<Title> = TitleReplacement
<Artist> = Foo
<Artist: Guest> = Bar
<Title> = Title (feat. Bar)
This is shown below. I cannot complete the Replacement because <Artist: Guest> is not selectable among the fields available within the "Replace with:" box.