Hello Curt,
The thing is that RadComboBox (the editing element of GridViewComboBoxColumn) searches for its selected value only once in the source and stays blank if it does not find it in this attempt. What you can try is to set the ItemsSource of the grid only after the one of the column is loaded completely. However, the most appropriate implementation depends entirely on your settings and structure of the application.
All the best,
Maya
the Telerik team
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