Попробуйте 'autocomplete' => 'off'
в опциях выбора даты:
<?= DatePicker::widget([
'type' => DatePicker::TYPE_RANGE,
'model' => $model,
'attribute' => 'tgl_mulai',
'attribute2' => 'tgl_selesai',
'options' => ['placeholder' => 'Start date','autocomplete' => 'off'],
'options2' => ['placeholder' => 'End date'],
'form' => $form,
'pluginOptions' => [
'format' => 'dd-mm-yyyy',
'autoclose' => true,
]
]); ?>