As of the latest release (10/04/2019) RadPieChart with a PieSeries having LabelBinding set to a property name, and ShowLabels set to True, with the list of data bound to ItemsSource containing one or more empty pie slices (a null in the list), causes a System.Reflection.TargetException to be thrown on load of the page containing the chart in iOS (this scenario works fine in UWP).
I am using the latest available Visual Studio 2019 (16.3.5) and the latest available Xamarin.Forms package (126.96.36.1998062) and testing on an iPad with iOS 13.1. The reason I need to have a null value in the list is so that the custom color palette I'm applying will always apply the same colors to the same data points (pie slices), even though in some cases, zero or one or two of the three pie slices will have no data value (and so should not appear in the chart, but still "use up" the corresponding color in the 3 colors of the palette).
In the 08/02/2019 release, this scenario worked fine in iOS (the 09/13/2019 release had the https://feedback.telerik.com/xamarin/1431067-chart-ios-invalidcastexception-is-thrown-when-the-chart-is-loaded issue, which is now resolved in the 10/04/2019 release, but which prevented my testing in iOS since in my app I had RadCartesianCharts on the same pages as the RadPieCharts).
The newly broken functionality in iOS could possibly be related to two other items that are marked completed:
https://feedback.telerik.com/xamarin/1366403-piechart-tooltip-behavior-is-not-working-properly (in 10/04/2019 release)
https://feedback.telerik.com/xamarin/1365842-chart-null-value-support (in R3 2018 SP release)