Group of features not working with MarkerCluster

Problem/Motivation

In our D7 site I submit all the markers in a group so they appear as a layer in the layer switcher that the user can turn on / off.

Using D9 with Leaflet 2.x.19, that works if I just use the Leaflet module, but once I enable the Leaflet MarkerCluster module there are two problems:

A. - The group layer isn't shown in the layer switcher

B. - The markers in the group are not clustered

Steps to reproduce

The code that builds the array of markers / points that are passed in to leafletRenderMap() ends:

    // We used to do:
    //   return $points;
    // Instead, return them as a group.
    // Then they're displayed as a layer in the layer switcher so
    // so they can be toggled on & off
    $group_of_points[] = array(
      'group' => true,
      'features' => $points,
      'label' => 'Markers',
    );

Proposed resolution

A. - The group layer isn't shown in the layer switcher

The code in leaflet.drupal.js that works is at line 247:

// Add the group to the layer switcher.
 self.add_overlay(feature.label, lGroup, false, mapid);

The code in leaflet_markercluster.drupal.js that doesn't work is at line 35:

// @todo we need to correctly handle the groups here
 cluster_layer.addLayer(lGroup);

I changed it to use the code from leaflet.drupal.js and now the group layer is shown in the layer switcher. ('self' isn't defined, so I used 'this' instead)

        // Add the group to the layer switcher.
        this.add_overlay(feature.label, lGroup, false, mapid);

B. - The markers in the group are not clustered

The code to handle groups in leaflet_markercluster.drupal.js at line 19 creates a regular layer for all the points in the group:

var lGroup = this.create_feature_group(feature);

Changing that to use a clustering layer shows the markers clustered correctly:

var lGroup = new L.MarkerClusterGroup(leaflet_markercluster_options);