Datatable two header rows not exported to pdf

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PHP

jQuery

DataTable

CodeIgniter

Hello,

My table has two header rows, DataTable is showing bother header rows but not exported to pdf please help.

My table is:

<table id="example" class="table table-striped table-sm table-bordered nowrap" style="width:100%">
 <thead>
	<tr><th colspan="2"><?php if(isset($session)){ echo $session; } ?> - <?php if(isset($class)){ echo $class; } ?>
			- <?php if(isset($section)){ echo $section; } ?></th></tr>
		<tr>
			<th class="text-primary">Roll #</th>
			<th class="text-primary">Student Name</th>
		</tr>
	</thead>
	<?php if(isset($rows)) { ?>
	<tbody>
		<?php foreach($rows as $row) { ?>		  
			<tr>
				<td><?=$row['rollno'];?></td>
				<td><?=$row['studentname'];?></td>
			</tr>
	  <?php  } ?>
	</tbody>
	<?php  } ?>
</table>

DataTable Image

PDF Output
Please help, thanks in advance.

What I have tried:

$('#example').DataTable({
        header: true,
        dom: 'Blfrtip',
        search: true,
            iDisplayLength: 10,
            lengthMenu: [
            [ 10, 25, 50,100, -1 ],
            [ '10 rows', '25 rows', '50 rows','100 rows', 'Show all' ]
            ],
            buttons: [
            'copyHtml5',
            'excelHtml5',
            'csvHtml5',
            {
                extend: 'pdfHtml5',
                orientation: 'landscape',
                pageSize: 'LEGAL'
            }
            ]
         });