Bug in select2 advanced when adding custom class name to container?

I am populating a select2 advanced field using the below code but it just shows normal select instead of select2 with search box. When removing the "containerCssClass", it works.

array(
                'name'            => 'Select Language',
                'id'              => 'language',
                'type'            => 'select_advanced',
                'ajax'       => true,
                'options'         => [
                    'java'       => 'Java',
                    'javascript' => 'JavaScript',
                    'php'        => 'PHP',
                    'kotlin'     => 'Kotlin',
                    'swift'      => 'Swift',
                    'javha'       => 'Java',
                    'javhascript' => 'JavaScript',
                    'pfhp'        => 'PHP',
                    'kottlin'     => 'Kotlin',
                    'swiyjft'      => 'Swift',
                ],
                'placeholder'     => 'Select an Item',
                'js_options'      => [
                    'containerCssClass' => 'my-custom-class',
                ],
            ),

this works

array(
                'name'            => 'Select Language',
                'id'              => 'language',
                'type'            => 'select_advanced',
                'ajax'       => true,
                'options'         => [
                    'java'       => 'Java',
                    'javascript' => 'JavaScript',
                    'php'        => 'PHP',
                    'kotlin'     => 'Kotlin',
                    'swift'      => 'Swift',
                    'javha'       => 'Java',
                    'javhascript' => 'JavaScript',
                    'pfhp'        => 'PHP',
                    'kottlin'     => 'Kotlin',
                    'swiyjft'      => 'Swift',
                ],
                'placeholder'     => 'Select an Item',
            ),