By default UWP's native RadPieChart clips its content to restrict the elements to render outside its rectangle. So, what we could suggest you for you scenario is set the ClipToBounds property of the native Chart to False. That you could achieve by creating a custom PieChartRenderer for UWP and inside the UpdateNativeControl method set the mentioned above property:
CustomPieChartRenderer : PieChartRenderer
Hope this helps.
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