As far as everything I have seen regarding the Guardians in Opulence, the crush mechanic should be on the side of the arm that is raised. On the volatile wing last night it was a frontal cone.
It can raise both arms to crush in a frontal cone. If only one arm is raised and you still see a frontal cone, then yes maybe it’s a bug.
Yes it was always one arm as far as I could see. I started going to the side opposite the raised arm and was stunned every time. You had to be behind the mob and you could see the cone out the front 100% of the time.