and it occurred to me that it seems to have acquired a negative connotation.

Yes, it has acquired some pejorative connotations, mainly as a result of its being used by representatives of various governement when they are referring to other governments or states with whom they have a difference of opinion. I am not if it will lose these new connotations or if they will stick around. (Come to think of it, it might be the result of Ancien Regime being wholly negative.)

Ceci n'est pas un seing.