After reading your reply, I try again and no it doesn't work...
If there is no mic, the side mount will point and any angle when release without tension.
But if there is mic, the side mount will not balance when I point at any angle other than horizontal without tension... for example, if I point up and release my hand without tension, the lens will continue moving up until the camera body hit the gimbal base.
Anyway, it'll work if I applied a bit of tension, but sometime you forgot to applied the tension in the field (especially after some actions) and release the camera...
Did you rebalance the load after adding the mic? It's lever, load and fulcrum from high school physics. Adding or removing weight anywhere other than at the center of mass changes the position of the center of mass for the rig, and if the center of mass isn't at the fulcrum, the load isn't balanced. If you put a mic or flash on the camera body, you need to loosen the clamp, slide the plate/foot back slightly to rebalance, then tighten the clamp again.
As long as you've got the right plate/foot for your lens, you should be able to balance the load properly so it moves freely and holds position at any angle.
I suppose there may be some design quirk of the BWG-Pro that precludes achieving proper balance, I haven't used one. I have no problem achieving proper balance with my RRS side gimbal, same thing on the 4th Gen Mongoose and Wimberley Sidemount gimbal heads that I've used. I prefer the RRS version over the other two because the lateral positioning is adjustable; with the Mongoose and Sidemount, you need to use a low-profile foot with large lenses like my 600 II, and the low-profile feet don't make good handles for hand-carrying the lens.