Create a report per Worker ID with different datasets feeding different tablix

Hi vla2a ,

Thanks for reaching out to Microsoft Fabric Community.

You are right that semantic models are a Power BI Desktop and Service concept and are not something you can create within Power BI Report Builder.

In Report Builder, each dataset is independent and a data region such as a tablix can be bound to only one dataset. Because of that, when you place a nested tablix inside a list or another tablix, it does not maintain a separate dataset context, which is why your Earnings tablix is switching to the Header dataset.

To work across datasets, you need to use supported patterns such as Lookup or LookupSet functions, or pass parameters into subreports.

Reference:
Report Builder functions - Lookup function in a paginated report (Report Builder) - Microsoft Report Builder & Power BI Report Builder | Microsoft Learn
LookupSet function in a paginated report - Microsoft Report Builder & Power BI Report Builder | Microsoft Learn

Another option is to shape the data at the source so that Header, Earnings, and Deductions are returned in a single dataset. This is generally the most stable approach for paginated reports and aligns with Microsoft guidance on preparing data for reports:
Data retrieval guidance for paginated reports - Power BI | Microsoft Learn

So in your case, you can keep the Header dataset as the main grouping by WorkerID and use LookupSet to bring in related Earnings and Deductions rows, or use subreports with WorkerID as a parameter. If modifying the Oracle query is possible, combining the data into one dataset should simplify the layout and grouping.

Hope this helps. Please reach out for further assistance.
Thank you.