This is a bug. Banking is not handled correctly by sub-spline, as it should.
I already fixed for next update.
Banking value can be used on expressions using: XSplineXDirection, XSplineYDirection, XSplineZDirection.
XSplineXDirection is the tangent vector over the spline, XSplineZDirection is the up vector, and XSplineYDirection perpendicular to both.
Banking angle can be computed from vectors using this expression:
# Banking angle to local XY plane. Limited to [-180,180] (0 = vertical)
vector z0 = [0, 0, 1];
vector vz = XSplineZDirection;
real angle = radToDeg(atan2(length(crossprod(z0, vz)), z0 * vz));
I've attached a (completely useless) example, where Segment Z size is proportional to the banking angle.