The (---) notation in live preview is rendered as it is in reading view, a horizontal line, most of the time.
But when it is a list item not on the first indent level it stays text.
Here’s a gif of it in reading, then source, then preview mode.
I thought that it was unusual behaviour, therefore a bug. Why a feature request? Also, even if this has to be called a feature request, can’t just this post be turned into one?
By that token I’d consider “horizontal lines in live preview” implemented, and this edge case as “something not working as expected”, making it a bug. But thank you for the clarification.