Metadata tagging in the mp4 container format used by Apple is very rigid, and IIRC it doesn't support multiple fields with the same name, or multiple values wit

Metadata tagging in the mp4 container format used by Apple is very rigid, and IIRC it doesn't support multiple fields with the same name, or multiple values within a field, so they can't really implement it. (They could implement it for most other formats, of course, but that would make their own formats look bad...)