Hi Rocko,
I am not able to reproduce this problem in the below sample that uses the same property settings that you have used for combobox setup in the GridGroupingControl.
Here is the sample download link
http://websamples.syncfusion.com/samples/Grid.Windows/F62381/main.htmAre you seeing any issues with the above sample? Let me know if this helps.
Thanks for using Syncfusion Products.
Have a nice time.
Regards,
Rajagopal