Timeline plus subgroup falling off the group

The subgroup falls into the next group at certain zoom levels or timeline pan.

HTML

<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/vis/4.21.0/vis.min.css">
<script src="https://cdnjs.cloudflare.com/ajax/libs/vis/4.21.0/vis.js"></script>
<div id="visualization"></div>

JavaScript

var items = new vis.DataSet({
      type: {
        start: 'ISODate',
        end: 'ISODate'
      }
    });
    var groups = new vis.DataSet([{
      id: 'all',
      content: 'bar',
      subgroupStack: {
        'sg_2': true,
        'sg_3': true
      }
    }, {
      id: 'second',
      content: 'foo',
      subgroupStack: {
        'sg_2': true,
        'sg_3': true
      }
    }]);

    // add items to the DataSet
    items.add([{
        id: 0,
        content: 'subgroup1_1',
        start: '2014-01-23T12:00:00',
        end: '2014-01-26T12:00:00',
        group: 'second',
        subgroup: 'sg_3'
      },
      {
        id: 1,
        content: 'subgroup2',
        start: '2014-01-26T12:00:01',
        end: '2014-01-29T12:00:00',
        group: 'all',
        subgroup: 'sg_2'
      },
      {
        id: 3,
        content: 'subgroup1_2',
        start: '2014-01-23T12:00:00',
        end: '2014-01-26T12:00:00',
        group: 'second',
        subgroup: 'sg_3'
      },
      {
        id: 4,
        content: 'subgroup1_3',
        start: '2014-01-26T12:00:01',
        end: '2014-01-29T12:00:00',
        group: 'second',
        subgroup: 'sg_3'
      },
      {
        id: 5,
        content: 'sg_3',
        start: '2014-01-23T12:00:00',
        type: 'background',
        end: '2014-01-29T12:00:00',
        group: 'second',
        subgroup: 'sg_3'
      },
      {
        id: 6,
        content: 'sg_2',
        start: '2014-01-25T12:00:00',
        end: '2014-01-29T12:00:00',
        type: 'background',
        group: 'all',
        subgroup: 'sg_2'
      },

    ]);

    var container = document.getElementById('visualization');

    var options = {
      start: '2014-01-10',
      end: '2014-02-10',
      editable: true,
      stack: true,
      stackSubgroups: true
    };
    var timeline = new vis.Timeline(container, items, groups, options);