If GroupKeepTogether is st to True, the Reporting engine will attempt to keep the entire group on the current page. If this is not possible, it will try to move the entire group to a new page. If the latter is not possible (e.g. the group needs more than one page), the group will be rendered starting from the current page.
I suggest to check the group and the detail section PageBreak properties and assure they are set to None.
Based on the available information I cannot say what is the exact reason for the described behavior. You may send us the report for an investigation to try to clarify it.
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