Theory – If the file beeing copied has not been reorganized or copied in key order, it will not be in key order when copied sequentially . Either reorg the file first to be in key order then copy to IFS to see if that corrects the problem.
I’m not aware of any definition of CPYTOIMPF that says order will be maintained. Nor can I think of any way to guarantee order in any case without either processing by key or through an ORDER BY clause. Since CPYTOIMPF uses neither, I’d say that you probably should use a method that guarantees order if order is important.