Unfortunately, I was not able to reproduce the issue reported. I have prepared a sample page, which you can find attached.
While testing, I found out one major difference from your description - in the latest version Q1 2014 SP1 of Telerik UI for ASP.NET AJAX, when you set a global RenderMode "Auto" key in the web.config, RadComboBox is rendered in Lightweight render mode, not in Native render mode. This can be verified by the label just above the RadComboBox on the sample page. It displays the ResolvedRenderMode of the control.
Could you, please, specify the version you use? And in order to assist you in the most efficient way I would kindly suggest that you provide a very simplified runnable project (aspx page, codebehind and datasource) that demonstrates the behavior that you describe. Thus, we could test it locally and determine what might be causing the issue that you are experiencing.
Thank you in advance for your cooperation.
Check out the Telerik Platform - the only platform that combines a rich set of UI tools with powerful cloud services to develop web, hybrid and native mobile apps.