Hi Sascha,
Thanks for using Syncfusion products.
I am afraid that I am not able to reproduce the issue that you have mentioned here. I set AutoFormat as “Office 2007 Blue”, and grouped the columns, but there is no frames added for the column headers.
Please refer the screenshot from the below link, which illustrates this clearly.
http://www.syncfusion.com/support/user/uploads/frame_9cce25cc.pngPlease refer the Sample from the below link, which illustrates this:
http://www.syncfusion.com/support/user/uploads/Frames_601d4be4.zipPlease have a look at the above and try once again, if still the issue exists, could you please send us a simple sample to reproduce the issue? so that we could find out the cause of the issue very soon and provide you a better solution.
Regards,
Raji