The problem isn't with the tags or the mappings. They are correct, and in the main view they sort correctly. However, in the column browser, "XXXX abcd" is sorted before "XXXX" not after it, as it is in the main view.
For example "Jett, Joan and the Blackhearts" shows up BEFORE "Jett, Joan" in the column browser, but AFTER in the main view. After is correct, IMHO.
Also - in the tag editor you have to manually click the checkbox for the sort tags; for other tags, it's checked automatically when you edit the text box. But when I check the box manually, I can clear the tag and save it.