OK.
To make this work as you expect it to, you need to select "Joe" first, then "Unassigned", then you can F2 and edit.
If Unassigned is your starting point, then at that point, all unassigned files are selected for further filtering. If you select Joe, then Joe's unassigned files will be listed and you can select those and work on their tags, but not by F2ing "Unassigned" in the pane, as that is the start point of your selection and covers all unassigned files.
-marko.