New lines in headings or consecutive new lines are rendered incorrectly in CommonMark Markdown 1.2

When rendering to markdown/1.2, a new line is always rendered as two trailing spaces followed by a line break (a Markdown hard line break). This produces wrong Markdown in some contexts:

• In a heading: ATX headings can't span several lines, so the text after the new line leaves the heading. E.g. xwiki/2.1 = a\\\\b = (or Markdown # a<br>b) is rendered as # a + 2 spaces, then b on the next line, which becomes a separate paragraph.
• Consecutive new lines: e.g. xwiki/2.1 a\\\\\\\\b (or Markdown a<br><br>b) is rendered as a + 2 spaces, a line containing only 2 spaces, then b. A line with only spaces is a blank line, so the paragraph is split in two when parsed back.

Expected

The new lines are preserved inside the heading / paragraph (e.g. using <br/> where a hard line break can't be used, or a backslash hard line break).