Huh?
Well, I am as confused as you are. I have used Final Draft, every version, since 4.x.
I have never downloaded a demo, but I have bought every version since 4.x and have never had any kind of problem like the one that you are talking about. You do not normally have to set your own line spacing and "space before" for elements/paragraph styles. You can, if you want, but the program has defaults.
Like 99.9 percent of people who ask questions, you did not say Windows or Mac, nor give the version of your operating system. I do not know if this makes a difference in regard to this question, but I am pretty sure that FD8 does not work with some older versions of the Macintosh.
Here is the situation with the Windows version, which works at least back as far as Windows XP.
*Elements* is under FORMAT | Elements.
When you open Elements, you will see a list of paragraph styles (Elements).
Click Paragraph. I think you are talking about "Space Before." ("Space Before" has *0* as a choice. Within an element itself, the line spacing is always at least *1*; there is no *0* choice.) Of the commonly used Elements, only Parenthetical and Dialogue should have *0* for "Space Before." The other Elements generally have *1* for "Space Before."
Try loading one of the templates: FORMAT | Elements ... Apply a Template.
Maybe that will provide proper spacing. If not, try setting your own spacing and see if the program works properly.
Let us know your results.