PWA: preventDefault not working with beforeinstallprompt

I am testing on Chrome 70 on an Android device which should perfectly well prevent the AddToHomescreen prompt from showing. I cannot prevent the prompt or capture for later use. The prompt continues to show on every page load.

The textbox is filled and shows that the beforeinstallprompt event is loading. preventDefault on the event handler is not preventing the prompt.

Why??????

    <!doctype html>
    <html>
    <head>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width,initial-scale=1">
    <meta name="theme-color" content="#eaa103">
    <link rel="manifest" href="/pwa/manifest.json" />
    <title>Some App</title>
    <link rel="stylesheet" href="/pwa/css/bootstrap.min.css" />
    </head>
    <body>

    <div class="container">
        <testbox></testbox>
    </div>

    <script src="/pwa/js/jquery.js"></script>
    <script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js">

    </script>

    <script>
    var deferredPrompt;
    window.addEventListener('beforeinstallprompt', function(e) {
        // Prevent Chrome 67 and earlier from automatically showing the prompt
        e.preventDefault();
        // Stash the event so it can be triggered later.
        deferredPrompt = e;
        $("testbox").html("beforeinstallprompt loaded");
        return false;
    }); 
    </script>

    </body>
    </html>