I haven’t done any font editing but is this true in general? Sounds like some kind of application-defined constraint. You’re talking about quadratic splines with the shared approximating point between interpolating points, and probably quadratic Bézier, not B-splines, I would guess. Quadratic Bézier would do this if the application decides to constrain all tangents to be smooth, which is a choice, not a property of the spline. Quadratic B-splines do not in general have this property; you can move any of the control points and have local control of the curve that doesn’t extend beyond the two neighboring control points. The difference is that B-splines don’t have interpolating control points in general - the curve only approximates all the control points, and you have to use phantom points or duplicated points to ensure the curve touches them.