Select multiple files from multiple artists/albums, etc.
In the Tag Panel, if you select the drop-down arrow next to artist/album/track/year/comment, etc., it only shows the current values of those fields in the selected files.
However, if you click the drop-down next to Genre, it shows all available Genres instead of only the genres of the selected files.
I don't see that behavior in the Genre drop-down. If I select a group of files with one genre, then I see that genre, plus and in the list. If I select a group with multiple genres, then only and are in the list.
Perhaps it's dependent on the file type, but I see this with both Mp3 and FLAC files.
Under File > Options > Genres I have 'Show only user-defined genres' ticked and my user-defined genre list is blank. A couple of reasons to do this: You can't define more than one genre by using a drop-down box - you have to type at least a part of the entry anyway. And I find it much faster to simply enter the genre manually than use drop-down.
As to the behavior that you see, yes, I'm sure it's intentional.