Due to changes made in T17739 it is no longer possible to insert additional control-points at a specific position in subdivision-polygons.
In the original code, the polyline of a PlanarFigure was represented by a struct of a Point2D and the index of the controlpoint after which this polylinepoint is loacted. In T17739 this struct was replaced by a simple Point2D, so the correspondance of polyline-points to control-points is missing.
Thus, the PlanarFigureInteractor searches for the index of the polyline-element on which the cursor is located (e.g. 83), and tries to add the new control-point at position 83 (which is the index of the polyline-element and not the index of the control-point). As the figures might only have e.g. 6 control-points, the new point is appended instead of being inserted at the correct position.
Thus, instead of adding a point between two existing control-points, that point is added as a new control-point between the last and the first control-points...