I’ve run into this as well. A search for “curve offset” shows that the issue has been discussed for many years. From what I can tell, the behavior of the curve in Dynamo is dependent on how it was natively drawn in the host application. In other words, Dynamo is at the mercy of Civil 3D in this case. The array is a good example - all of the curves are planar, but obviously not all in the XY plane. There have been a lot of posts where people suggest adding a “plane” input to the Curve.Offset node in order to constrain the offset.
The only workaround I’ve found is to extrude the curve as a surface along the canonical Z axis vector, offset the surface, then intersect the surface with the XY plane. Another option is to just translate the curves by a normal vector in the XY plane, but that only mimics a true offset for line segments, not arcs.