Hi Cédric:

I am including Christof who did the markdown work.

This is a mismatch between simplemde's formatting and the back end.
I am not sure if simplemde needs a fix or the roundup backend needs
a fix, or if some user education needs to happen:

If some other mde editor is used on the front end, then we can
have the same issue but in the opposite direction.

Maybe this code needs to be conditional with a config option?

I have developed a test for this patch (that skips testing with
mistune), so we can include it if the discussion points that way.

Also there has been discussion of this as a misfeature:

  https://github.com/sparksuite/simplemde-markdown-editor/issues/71

and markdown's break format (per 
https://daringfireball.net/projects/markdown/syntax#block) is:

    When you do want to insert a <br /> break tag using Markdown,
    you end a line with two or more spaces, then type return.

so it looks like simplemde should have a "hard return" option that
inserts the trailing spaces.