The CSV rendering extension renders the Table item by expanding the table and creating a row and column for each row and column at the lowest level of detail. Due to the this data denormalization (flattening) some Table/Crosstab configurations, such as the one with nested tables, will not be exported correctly. This behavior is by design and due to CSV format specifics, so we cannot suggest a workaround at the moment.
Additional information on the topic is available in the Design Considerations for CSV Rendering
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