I just tried to see if I could make it do it on this post. I can't figure out what exactly is causing it.
I've tried using code tags not in BBCode Mode, code showed up fine. Tried using code tags in BBCode Mode and code showed up perfectly also. Tried editing the post and editing the code and it still showed up fine.
Here are the links to two posts that have it.
Maybe that will help you out. I'll play around with it for a couple of minutes to see if I can figure out what is causing it.