Kartik grouping option is not working in pdf

Question

I'm using Kartik gridview for grouping. However, grouping doesn't work while generating/exporting the pdf. ('group'=>true)

How to make the groups in the pdf?

[
   'headerOptions' => ['style' => 'color:# ;text-align:center;'],
   'attribute' => 'subject_name',                                            
   'contentOptions' => ['style' => 'min-width:180px;'],
   'format' => 'raw',                                                                
   'group' => true,
]                 `

I'm using the above code in Grid view, it groups the subject name in view file and also works fine in Kartik export, but "It does not work in the mpdf file when I'm using separate pdf file.

Thanks!