I know Laz and I recently went through something like this on the blog system (that he fixed nicely and quickly). This is quite the opposite, instead of adding lines, the forum seems to take one away when quoting.
Today I pasted in a quote from an interesting book I read. In the text there was the normal line break and paragraphs were spaced correctly.
The output, however, pressed the paragraphs together so that the next paragraph started on the next line without a break. It was easy to fix, I just edited the post and put another line in there, but I'd hope that wouldn't be needed.
Text came via a copy from the Calibre e-book viewer (Windows) if that helps. (Always there's a chance of GIGO)