What happens if you first add a contact to the address book, then add it to a mailing list by drag and drop from the right pane to the list icon in the left pane?

That works. Any idea why the normal method does not?

I added an address to a mailing list directly, then closed the list. The contact was automatically added to the containing address book, and was retained in the list when reopened. When address books don't keep changes or accept new entries, it sometimes indicates the book is corrupted. The usual fix is to export the book to an LDIF file, close TB, delete abook.sqlite (Personal Address Book) from the profile folder, restart TB, import the LDIF, then drag (move) the contacts from the imported book to Personal Address Book.

Thanks