Custom Field trasnlation with Polylang Pro

Hi,
I have the following code to register custom fields for translation (copied directly from the MetaBox code generator):

<?php
add_filter( 'rwmb_meta_boxes', 'your_prefix_register_meta_boxes' );

function your_prefix_register_meta_boxes( $meta_boxes ) {
    $prefix = '';

    $meta_boxes[] = [
        'post_types'  => ['chart-mapping'],
        'translation' => 'advanced',
        'title'       => __( 'test mapping fields', 'your-text-domain' ),
        'id'          => 'test-mapping-fields',
        'fields'      => [
            [
                'type'        => 'textarea',
                'id'          => $prefix . 'translateable',
                'name'        => __( 'translateable', 'your-text-domain' ),
                'translation' => 'translate',
            ],
            [
                'type'        => 'text',
                'id'          => $prefix . 'untranslateable',
                'name'        => __( 'untranslateable', 'your-text-domain' ),
                'translation' => 'copy',
            ],
        ],
    ];

    return $meta_boxes;
}

The option 'translation' => 'translate' does not work correctly in the textarea field, and always behaves in the same way as a 'copy'.
Note this is the same if the top-level option is set to 'translation => 'translate' instead of 'advanced' as well.

Is this a bug or am I doing something wrong?