Creating server controls inside postback event handler is not proper approach since this is a late stage in the page lifecycle to create controls (their viewstate/lifecycle will be corrupted and they will disappear on subsequent postbacks). You will experience the same behavior with regular MS Button or Calendar for example if you create them in the same way.
Please revise your logic to address the issue. After that, to locate the controls after postback, use the FindControl(id)
method of the framework by passing their ids as argument.
the Telerik team