It is known limitation of the new rendering of RadInputControl. Check these blogs:
When you load the page again, the browser itself populate these values, but the internal value of the control is the old one, and it does not use the visible value as actual value.
We are researching on possible ways for fixing this, but with no success at this moment. As alternative of the TextBox you can use RadInputManager with Asp:TextBox. The Manager works entirely differently, and can handle these cases.
the Telerik team
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