The Extension of the outer Table borders is drawn to indicate that the table continues on the next page. It will appear in case the height of the detail Table cell is at least two text lines and there is enough space on the page for (at least) the first line, but not enough for (at least) the last line.
In the context of the above explanations. one workaround would be to set the design time height of the cell/TextBox to less than two text lines.
Another workaround would be to set the left and right borders of the Table item to None.
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