The issue may be related to the width required by the content of the Report Header and Footer sections.
The Report Header will be fit at the beginning of the first column of the multi-column detail section, whereas the Report Footer will be placed at the bottom of the last rendered column, i.e. they will *not* be rendered across the entire page width. Therefore, if in one of these sections there is content (TextBoxes or other report items) with location surpassing the width of the detail column, this will force horizontal page break.
You may test fitting the content of the Report Header/Footer of the base report in a smaller area (taking into account detail column width of the inheriting reports).
I have attached a sample project mimicking the described approach that works correctly on our side.
If the problem persists you may open a Support ticket and attached the project for local investigation.
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