Conflict between WYSIWYG and Line break converter
By default Drupal uses the Line break converter to make it easier on authors to enter text. This filter looks for blank lines in a node, and turns the neighboring blocks of text into paragraphs. This makes it so an author simply enters a blank line between paragraphs and it's all very simple and straightforward.
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.