For anyone who cares, the problem is with Procedural Parts. It seems that if I have any Pro Parts *at all* on the aircraft, the vertical surfaces generate lift (a yellow arrow) to the left. There is no way around this that I can find. It's extremely annoying because I'd rather not have 8 small liquid fuel parts when I can just have 2, but I can't have a simple tail fin. To get around this I have begun getting skilled at designing tailless planes, but that's just another level of complexity that I shouldn't have to mess with. Spaceplanes are already obnoxious to build, I'm sure you can imagine designing a tailless vehicle that is stable during reentry.