It is not a bug. Notice that the order of the numerator has changed from V[B]-V[N] to V[N]-V[B] with a corresponding change of sign of the overall subexpression. V[B]-V[N] = -(V[N]-V[B])
Why? Because it felt like it. Symbolic expressions may be re-ordered whenever they are not held, and when you used the evalAt operator you unheld the expressions.
