Hi there, and welcome to the forum,
I've modified one of the examples from the RibbonX book, and the result I'm getting is exactly what you're seeing.
If I recall correctly on this, this is one of the aspects that MS could do that we can't. One of my co-authors wrote this chapter of the book though, so I'll ping him and see if I'm off base on that.
(Sample file is attached, just so I don't have to re-create it later.)