Error using matrices in Mathcad Prime 3.1

I'm trying to compute the stress in a thick walled cylinder using a matrix that has two pressure values stored in it. When my supervisor and I were going over the calculations, we noticed that one form of our von Mises stress formula will give the correct answers, while another equivalent form of the exact same formula will give erroneus values. However, when I set the pressure variable to just one of the values that are included in the matrix, it computes it correctly. Is this a bug with Mathcad, or am I doing something wrong?