Hi Helmut,
Thank you for contacting Syncfusion Support.
We have checked the reported behavior with "Splitbuttons are not rendered properly in latest themes with Ribbon" and we too able to reproduce it. We have forward this to development to validate this behavior. We will validate and update you the complete details on 17th Nov, 2020.
Please let us know if you need any further assistance on this.
Regards,
Vijayalakshmi VR