Unsaved changes in metabox inputs not detected in the block editor

Question

I have added a custom field to the regular WP post type and added a meta box to the post editor as below (it is just an example with trivial checkbox):

add_action( 'add_meta_boxes', 'add_meta_box_my_custom_field' );
function add_meta_box_my_custom_field()
{
    add_meta_box(
        'my_custom_field',                          // $id
        __( 'My Custom Field', 'mycustomfield' ),   // $title
        'display_my_custom_field_meta_box',         // $callback
        'post',                                     // $screen
        'side',                                     // $context
        'low',                                      // $priority
    );
}

add_action( 'save_post', 'save_my_custom_field_value' );
function save_my_custom_field_value($post_id)
{
    if( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) return;
    if( !current_user_can( 'edit_post' ) ) return;

    $new_value = isset( $_POST['my_custom_field'] ) ? 'yes' : 'no';
    update_post_meta( $post_id, 'my_custom_field', $new_value );

}

function display_my_custom_field_meta_box(){
    global $post;
    $check=get_post_custom( $post->ID );
    $checked_value = isset( $check['my_custom_field'] ) ? esc_attr( $check['my_custom_field'][0] ) : 'no';
    ?>

    <label for="my_custom_field"><?php _e( 'My Custom Field', 'mycustomfield' ); ?>:</label>
    <input type="checkbox" name="my_custom_field" id="my_custom_field" <?php if($checked_value=="yes"){echo "checked=checked"; } ?> >
    <p><em><?php _e( 'Enable My Custom Field.', 'mycustomfield' ); ?></em></p>
    <?php
}

It works fine in standard WP installation with block editor etc. The only thing that does not seem to work is the detection of unsaved changes.

When I open the post to edit it, change current setting of 'My Custom Field' and try to navigate out of the page, I do not get the warning. Unsaved changes are lost.

I have played with changing the priorities, different hooks, but got eventually clueless. Thus, it must be something really trivial that I am missing. Can anybody provide a hint?