No, you’re correct. It will make the frontmatter show up in normal preview, but not on “hover preview”. (Which I quite like, in order not to confuse my or any users.)
Seems I misunderstood you re hover preview, sorry.
If you really want that: You could probably style it using a CSS snippet:
.markdown-embed-content pre.frontmatter.language-yaml {
display: block;
}
Specificity is rather high, so you might get around without using !important.
Is that what you were out for?