Generally transformations applied to the shapes can be considered as an unsupported feature. When applied - they effect manipulations (rotate, resize), dragdrop, export, settings pane values etc. We are declining this bug fix as it is related to this unsupported functionality. Diagram is designed in such way that such transformations should be performed with the Diagram API - shapes rotation angle and size, the geometry will stretch accordingly in scale scenarios.

For the specific ExportToImage issue, here are two possible solutions:

1) Set the enclosing bounds parameter explicitly (to avoid being calculated internally): 

  diagram.ExportToImage(stream,
                       enclosingBounds: new Rect(new Size(this.diagram.Width, this.diagram.Height)),
                        backgroundBrush: new SolidColorBrush(Colors.PaleGreen),
                        margin: new(20));

2) Use our ExportExtension methods:

            Telerik.Windows.Media.Imaging.ExportExtensions.ExportToImage(
                 this.diagram, stream, new System.Windows.Media.Imaging.PngBitmapEncoder());