Note that this really only applies when you're pasting a single block of text, essentially something without a line break. That will be pasted inline if possible. If you have a line break or a paragraph break, the content will very likely come through with a line break before the first pasted content. Proper formatting of the pasted content more or less ends up causing this.
I noticed something that may or may not be related to this.
When I make a post here and then highlight the entire text or just a section of a post, then paste that into another (XF1.x) forum, the result add in double linebreaks between paragraphs that I have to manually remove.