When you fetch the coalesce of arguments/attributes/parameters which do not exist in the select statement it will throw an error of invalid context which is SQLCODE = -206, which means the coalesce function is trying to find the column which does not exist in the view.
Also, I’m not sure if I understand the situation you describe correctly. I tried replicating a similar scenario, according to the description you are removing the 2 input parameters (dateparam1 and dateparam2) from the output, but those are mandatory and cannot be removed. If you can provide more details and clarify this it would help to understand the situation. In any case, if you are a customer, you can file a customer support case and provide further details.
Hope this helps!