fblome
Here are screenshots of the editor and HTML preview.
After looking at the "Inline line break syntax" section (just up the page from the hardbreaks section in your reference), I see that they are inserting a space, a plus sign, then a return. I was reading the manual reference as only requiring a space and a plus sign.
I've been writing most of my documentation in Word and Pages until now and for those of us coming from those tools, a wee bit more detail is helpful.
The Asciidoctor site is comprehensive but intimidating. Trying to find what I want is often frustrating. I'll look for it under one term, when they call it another.
So you can chalk this one up to not referring back to the source documentation.
Cheers,
Jon