Vector Rotate Error

Can anybody help me out with this problem?

I am working on building elevation algorithm as you can see in the attached file.

To make vertical mullions symmetric, I array the half of the mullions from the center of each side of a rectangular, and mirror (here I am using 'rotate') them.

When I apply my algorithm to Y-axis it works, as you can see.

But, it turns out the mullions flipped in X-axis.

I also tried Vector Rotate with angle and when the angle becomes 1.0 pi, it generates flipped mullions.

Can anybody advise me of solutions with explanations?

Many thanks in advance.

Esbi