Hi Richard,
Thanks for posting the query to us.
I am afraid that I am not able to reproduce the issue of “Format of GridGrouping control not being properly displayed during postback when modal dialog appears”. Please refer the sample in the link below:
http://websamples.syncfusion.com/samples/Grid.Web/6.1.0.34/Gridgrouping/main.htm Please try running the sample and if still the issue exists could you please try reproducing it in the above sample and send us the modified sample so that we could sort out the cause of the issue and provide you a solution?
Regards,
Janagan.