Hi Geffrard,
Thanks for your interest in Syncfusion products.
We could able to understand your scenario and this reported scenario is well known query in our side. Currently the GridGroupingControl does not have CodedUI support for nested table cells. So already we have logged the feature report for this scenario. The reported feature will be implemented in any of our upcoming releases and we will let you know once the feature has been impemented.
Thanks for your understandings.
Regards,
Piruthiviraj