Lookup_Selecting is server event side to add a filter, it has nothing to do with AutoFill (which is JavaScript). However, if your Lookup_Selecting server event filter out the options so there is no option available, there is nothing to fill. If your Lookup_Selecting server event caused server side error so there is no valid response from server, there is nothing to fill either. You may check HTTP response and see if there is any server/client side errors.