This is NOT an bug, just the way curve frames work. Curvature frames are created by the tangent vector (which winds up being the X vector of the plane), the curve normal (the direction created by the point on the curve to the center of the osculating circle that describes the curvature at that point... this becomes the Y vector), and the bi-normal (which is just the cross product between the tangent and curve normal). What your seeing is that the curvature of the curve is changing from one "side" to the other. Therefore, the orientaiton of the frame changes.