Hi Harald
Thank you for contacting Syncfusion Support
In ButtonAdv control, we have maintained inner border with LinearGradient BorderBrush. Due to this Transparent wasn’t applied for ButtonAdv BorderBrush. We have prepared workaround sample for this. In this sample, we have overridden our default ButtonAdv style to customize our ButtonAdv as per your requirement.
Please let us know if the sample helps.
Regards,
Mariappan S