Your code looks fine and this is the correct way to retrieve custom attribute's value. I have created a small samples based on the provided information. However I was not able to reproduce the same behavior as you. For your reference I'm attaching my sample. Please check it out and let me know if I have missed something.
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed