That banking effect is caused when the base spline changes direction and vertical angle on the same corner. In order for the mitred geometry to align correctly at the corner, the cross section must be rotated. As far as I know it's not possible to bevel that corner in a way that would connect the corner correctly while rotating the segment so that it remain flat. This is also true in real-life. You can see in the image below that Loft and RailClone both handle this the same way:
RailClone does however have a vertical
mode, found in the Segment's settings. This will force the segment to remain vertical, deforming geometry where necessary. This will remove the banking effect, but you will find that corners twist to keep the geometry flat.
The easiest way to fix these issues therefore is to slightly modify the base spline so that it's either flat while turning the corner...
Or curve the corners instead of bevelling them. You can use the Generator's Rules > Fillet Radius
setting to do this automatically ...
You'll notice a lot of balustrades on stairs use these two techniques to overcome the same limitations.
I hope that helps, please let me know if you have any further questions.