Hello Andrew Zhao,
I tried to use the code snippets that you provided to create a sample page and replicate the issue on our side but unfortunately without success.
However I created a basic sample to illustrate the approach of adding RadComboBoxItems server-side and make it easier for you to implement similar logic at your application.
If the issue persists - could you please provide us the full code of your page?
Feel free to contact us if you have additional questions.
All the best,
the Telerik team
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