Under the covers these pseudo-WYSIWYG editors all edit HTML code.
Most (or all) of these pseudo-WYSIWYG editors do not understand the significance of the blank lines. They do not render blank line separated paragraphs as paragraphs, but instead run the paragraphs together.
This means a site with existing content written using Line break converter will be unable to accomodate the pseudo-WYSIWYG editors. Or, if such a site wants to use them, it must convert its content so the blank lines are replaced with <p> tags.