You have num:2, so subsubsections are not TOC'ed, so they don't get
the alternative. If you set it to 3, all should work.
That is what we have been discussing. There are situations where you do not want a headline to appear in TOC, but still want the ALT_TITLE used. It is now possible in org-mode, but there was some bug because of which it did not always work.
Nicholas has just fixed it, but, I think, there is still an extra * appearing.