I tried to reproduce your scenario but I wasn't able to. Can you please have a look at the sample project I attached and let me know if I am missing something? If you can modify the solution to better illustrate your scenario, we will be able to further investigate the case.
Also, if you want to use a control that allows controlled input and validation of the entered values, then you can take advantage of the RadMaskedInput
suite of controls and their RegularExpression Validation
the Telerik team
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