You have to press the spacebar on empty lines for them to display. For example, if you press return twice (and see a white space line in the editor), then when you click Submit, the white space won't show up in the forum thread.

However, if you press return, then press the spacebar, and then press return/Enter again, then you see the white space line. It looks like this:
This is by design to eliminate unnecessary white space.

NOTE: If you click Edit, the space on the empty white line disappears, so that when you click Submit again, the white space line disappears from the thread. So after you click Edit, you need to press the spacebar and add a space back on the white line again before you click Submit.