I suspect it may be for the same reason that calibre cannot modify any
other metadata in the KF8 portion of a dual-MOBI.
i.e. As far as the Kindle Voyage knows (since it only looks at the KF8 section), the ebook in question does not have an ASIN.
(If it doesn't show as one of those nasty "Personal" documents, that would be because I assume you also have the output preferences set correctly.)
tl;dr
dual-MOBI is evil unless you actually have a need for it.