Hello,
Whereas uStore indeed picks many attributes from the designated uProduce "Template" job, we have always seen the details related to the Data Source (or Recipient List) simply dropped, when a uStore Order comes, as they are simply replaced by the uStore Order's Recipient List.
So, everything from uProduce such as a SQL query or a Plan Filter that would sort the records, Limit records range or Split to multiple jobs are not taken into account by the regular "Process (All Copies)" Action in uStore Admin.
I do not think that you are doing wrong.
If we were to achieve what you are trying to do, we would consider developping a Custom Order Handling Action, certainly called "Process (Split)".