As my colleague Steve said, the multi-column report by design have this rendering behavior (from left to right, top to bottom) and applying a PageBreak property
at the end of each group will result in a new column, not a new page. Also, the group header section is also a part of this multi-column layout, so it would remain the same width (in your case of 2 column report, it will take only the half of the page).
Based on the provided information and example, this layout could be achieved with List item and indexed data starting from across the page and then down. For more information and example, check the How to: Create Multi-Column Report - Across the Page and Then Down
Then you could set PageBreak for each group to render on a new page.
In case of difficulties, please open a new support ticket and attach a sample report definition which we can investigate further and provide you more accurate suggestions.
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