Switching from Markdown to rich text mode strips blank lines in a variety of cases. For example writing this in Markdown mode:
<!-- some HTML comment -->
paragraph
has the empty line stripped when switching to rich text mode:
<!-- some HTML comment -->
paragraph
According to CommonMark specification blank lines don't break blocks, so the editor should keep blank lines when switching modes because the users may want to use them to visually break walls of text in the source.