You cannot have spaces directly inside constrained formatting marks.

You may want to study this section in the docs: https://asciidoctor.org/docs/user-manual/#formatting-marks

There are limits to what formatting you can nest (even though you can go pretty deep if you get creative). AsciiDoc is not a general purpose formatting language. It's a writing shorthand designed for standard formatting that covers most publishing needs. So if you ask it to do too much formatting, it's going to look noisy.

-Dan