The designer throws an error when the default style of a control which has a RadSplitButton/RadDropDownButton in its control template is extracted in a ResourceDictionary and then merged in App.xaml. There must be a custom style which is based on the extracted one to reproduce the error. This behavior is only present in 2019 R3.

The error is "This feature requires service 'Microsoft.Windows.Design.Services.ValueTranslationService' to be present, but it could not be located."