Why does my kml description balloon have no tail when displayed online on Google Earth?

Question

I am a kml "newbie" trying to attach a balloon to a placemark.

The default behavior of a balloon associated with a placemark is to display text in a box with a tail pointing to the placemark's location after you click on the placemark. I import my kml files into Google Earth online to test. My balloons appear when I click on the placemarks, but I never get the "tail". Instead, the balloons appear in the upper left corner of the screen.

I've been reading about BallonStyle in the kml reference doc: https://developers.google.com/kml/documentation/kmlreference

I have copied and loaded examples from: https://developers.google.com/kml/documentation/extendeddata among other places, and the tail never appears.

I wrote this simple kml file that I think SHOULD use the default balloon behavior, but doesn't. The placemark has valid coordinates. What am I missing?

<?xml version="1.0" encoding="UTF-8"?>
<kml xmlns="http://www.opengis.net/kml/2.2">
    <Document>
        <Style id="Piper">
            <IconStyle>
                <color>ff0000ff</color>
            </IconStyle>
            <BalloonStyle>
                <text>$[description]</text>
            </BalloonStyle>
        </Style>
        <Placemark>
            <name>Piper 2018-08-01T16:45:16</name>
            <description>Accuracy = 10m</description>
            <styleUrl>#Piper</styleUrl>
            <Timestamp>
                <when>2018-08-01T16:45:16</when>
            </Timestamp>
            <Point>
                <coordinates>-110.766531000,43.851320000</coordinates>
            </Point>
        </Placemark>
    </Document>
</kml>

Thanks for any insights.