vis.js timeline items missing when group label is off screen

Question

I have encountered an issue using the vis.js timeline control that has me puzzled. I have my timeline setup with groups on the left hand side, and via the timeline options I've sized the timeline control so all the group + items are not visible at the same time, but the groups and items can be located using the vertical scrollbars. I use a little javascript to sync the group and items scrollbars locations.

I've found that if the content label of the group ID is not visible on page load, then that group's items will not be visible even when I scroll down to find the group listed.

I've prepared two examples to better illustrate the issue I am having. The two examples are identical except I've labeled one group "AAA Example" in the first example, and "zzz Example" in the second example. You will see that the item data does not display in the second example.

Example 1: http://www.pitchpipetuner.com/vis.js/example_1.html

Example 2: http://www.pitchpipetuner.com/vis.js/example_2.html

The options I've set on the timeline are:

    var options = {
        stack: false,
        multiselect: true,
        multiselectPerGroup: true,
        verticalScroll: true,
        horizontalScroll: true,
        maxHeight: "440px",
        height: "440px",
        orientation: "both",
        groupOrder: 'content' 
    };

Could someone explain how I could correct this issue? I've seen there is a Dataset property called limitSize, and I tried to set this to false but that did not seem to correct the issue.

I am using Firefox 57.

Thanks for any advice!