A small glitch appears when the pencil tool is used. The glitch is demonstrated in the attached image.
Thank you for the provided information. Based on it I've prepared a possible workaround. You can find it attached in this reply.
Technical Support Engineer
I've attached an updated version that shows how to set the StrokeMiterLimit via Style targeting RadDiagramShape instead of setting it in code. This is useful in case you use the serialization feature of the diagram.