There is no concept for grouping field in the detail section. In fact the behavior you observe is by design and duly noted in our documentation: Design Considerations for CSV Rendering
Group Header Section: Renders its content at the beginning of each detail row after the Report Header content if any.
The detail section renders a record for each detail row on a new line and this behavior is expected. Note that the main purpose of the CSV format is to transfer data between different information systems (programs). For example you may import it in MS Excel or Word. For this reason the CSV format is not designed to be human-readable and is not "WYSIWYG" as Pdf, Html or Excel formats. More about the idea that lies behind the CSV format can be read here.
In short, there is no way to "move" a field when exporting to a specific format, and if you need such functionality, our suggestion would be to create a second report with changed layout according to your needs, which would be used for exporting to excel.
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