When the DropDownPlacement property of the RadDropDownButton element is set to Top and the Windows 11 theme is used, a Setter in the default ControlTemplate tries to set the VerticalOffset of the drop-down Popup element. This is done via a converter (of the type of MultiplyValueConverter), to which a "-1" is passed as a ConverterParameter. In the Convert method of the converter, the native System.Convert.ToDouble method tries to convert the ConverterParameter to the type of double. Internally, the Convert.ToDouble method uses the current culture when converting a string to a double.

This is observed when using a different culture, for example, "ar-EG".