No, Lee. It is my mistake that my stupid brain not thinking about other {$GLOBALS['threadfields']['key']['value']} variable in postbit as soon as possible when I see your attachments. How stupid I am!!!
The reason why it happens to you is, the preview post calls postbit template. So, if we have the {$GLOBALS['threadfields']['key']['value']} variable in our postbit template, the conditional "can't block" it, because it is still in
newthread page. It's a bit hard for me to explain it
I've added other solution in above post if you encounter the same problem. Maybe more flexible and simple. But if the previous solution has solved the problem, I think it is not needed.