I just realized that if you add multiple same-named vorbis tags to multiple Flac files then you cannot later edit them all at once. You can only do it file by file.
If I have the following tags:
In the column display, only one ARTIST name is shown (the first one alphabetically?). If that one is edited, then all other ARTIST tags are all lost.
In the Tags dialog, if all files in the folder are selected, then only a single ARTIST tag is shown, with the value . If that tag is edited then all other ARTIST tags are lost. If only a single file is selected then all ARTIST tags are shown and can be edited.
Similarly in the Tag panel, if all files are selected is displayed and only a single ARTIST value is displayed in the drop down. Editing either results in all other ARTIST tags being deleted. If only a single file is chosen, then the convention of Arist_name1\\Artist_name_2 is displayed and can be edited or added to.
It seems logical when multiple files are selected, that if all files selected have the exact same values for the same number of same-named tags, then all of them should be displayed and should be editable. Only when there's a mismatch should be displayed.