Hi Subrata,
Thank you for using Syncfusion Products.
I have tested the issue in the following sample. But i am afraid I was not able to see the issue as you mentioned. Please see the same and if the issue still exists, then reproduce the issue in the below sample and send us the modified sample. So that we could analyze the issue here.
http://files.syncfusion.com/support/Grid.Windows/GDBGComboBoxIssue.zipRegards,
Jisha