You're better off filing a ticket for the rich text editor. This widget should render line breaks with the br tag and not with paragraphs. The document generator is treating the p tag correctly.
@Bas:
I tried to configure the Rich text editor to use BR instead of P. After that I get HTML like this:
<p>this is line one<br /><br />
this is line 2 after a blank line</p>
But when the user tries to center line two you get the following HTML:
<p style="text-align:center">this is line one<br /><br />
this is line 2 after a blank line</p>
And the whole paragraph is centered.