Referring to the knowledge base, "there are two groups of controls which are extended using the RadInputManager. The first are controls which are located directly on the page, such as a normal text box, somewhere on the form. The second are text boxes located in another control - for example a text box nested in a repeater.
I would suppose that this is an expected behavior -- that RadControls can't be used with RadInputManager.
tb = $find(
"<%= RadNumericTextBox.ClientID %>"