Copy some sub fields in group with polylang

Hi,
I need to copy some sub fields with polylang, not all fields of the group.
I used to copy needed custom fields with polylang using:

add_filter( 'pll_copy_post_metas', 'copy_post_metas' );
 
function copy_post_metas( $metas ) {
    return array_merge( $metas, array( 'my_post_meta' ) );
}

But sub fields are not registred as meta_key in wp_postmeta table, they are stored in an array in meta_value.
Is there a way to do that ?
Thank you