I believe the behavior is observed only in the UWP platform and in order to remove the additional effects you need to customize the appearance of the RadPieChart control for this platform. More specifically, you can apply the following Style in the UWP MainPage:
Basically, you need to modify the default styles applied to the control, which are available in the following GitHub repo - Telerik UI for UWP. You can examine the different styles and templates and apply further modifications if needed. I have attached a sample for your reference.
Have a great rest of the week.
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