Huh, that's weird. What if you write it elsewhere, then paste it onto the forum post template? (That may seem like an ignorant question but I'm not that tech-savvy and it's probably something I'd try.) Does it do the same thing in both "quick reply" and "advanced" mode?
I'd PM one of the moderators and ask this post be moved to the site feedback forum; you'd probably get more knowledgeable answers. It may be a known issue, even.