Bob, you're misunderstanding how playlist groups work. They don't match anything - it's just a container for other playlists. When clicking on a playlist group node, it shows all files that are part of all playlists under that node (so you can argue that it matches a list of playlist IDs, but that's all). The files do NOT need to have anything at all in common for filtering. You can simply drag a bunch of assorted files into a playlist, then drag it under a manually created playlist group. That's it, no filtering at all.
The bug is that you should be able to name the group anything at all without affecting the files listed under it.