I am writing again because I misunderstand your requirement. Please excuse me for that.
Actually, the reported behavior is caused by the fact that the RadDiagramShapes expose a StrokeDashArray property that is bound to the Path's (that represents the shape geometry) StrokeDashArray and its default value is set to "1, 0" (or new DoubleCollection(1, 0)).
To avoid the corner's clipping you can set the StrokeDashArray property to "Null"
or "new DoubleCollection(0)".
shape = new RadDiagramShape
StrokeDashArray = null,
Please try this and let me know if it works for you.
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