You can post it with the [ plain ] tag surrounding it.
Hah, that's funny. You are right of course. I never experienced that.
I won't fix it for a while though. I am in the process of porting the whole site codebase to XenForo version 2 (from version 1) and rewriting all parts of it. It will take some time, but I will be revisiting the [tabs] BB code and rest assured I will test it with signatures too at that time.
I am hoping to be done with the migration this year.