为啥WPF中插入的表格没有内边框线

为什么WPF中插入的表格没有内边框线
我在richtextbox中插入了一个表格，代码如下
<RichTextBox Margin="0,20,0,0">
            <FlowDocument>
                <Table BorderBrush="Black" BorderThickness="1" ClearFloaters="None" BreakColumnBefore="True">
                    <Table.Columns>
                        <TableColumn Width="25"/>
                        <TableColumn/>
                    </Table.Columns>
                    <TableRowGroup>
                        <TableRow>
                            <TableCell>
                                <Paragraph>&#x2665;</Paragraph>
                            </TableCell>
                            <TableCell>
                                <Paragraph>Raspberry</Paragraph>
                            </TableCell>
                        </TableRow>
                        <TableRow>
                            <TableCell>
                                <Paragraph>&#x2665;</Paragraph>
                            </TableCell>
                            <TableCell>
                                <Paragraph>Vanilla</Paragraph>
                            </TableCell>
                        </TableRow>
                    </TableRowGroup>
                </Table>
            </FlowDocument>
        </RichTextBox>
为什么出来的效果却只有外边框而没有内边框呢？效果如下：

可我要达到的效果是像word里面的表格一样啊，请问如何实现？
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