Google Maps Javascript api zoom issue with marker

I’ve a simple map with an svg file as marker icon. When the map is loaded no problem but, if i try to zoom in or out, the marker changes position (visually) and this is a problem.
The marker must not change position but remain fixed on the reference point.
How can I solve this problem ?

<style type="text/css">
    body { font: normal 10pt Helvetica, Arial; }
    #map { width: 100%; height: 100%; border: 0px; padding: 0px; }
</style>
<script src="https://maps.google.com/maps/api/js?libraries=geometry&key=AIzaSyBpL5_1SzkA3Q6LATyd19-8g5F_Zq_6w70" type="text/javascript"></script>
<script type="text/javascript">
    var icon = {
        url: "https://www.aisilbagno.it/wp-content/uploads/2015/04/google-maps-marker-for-residencelamontagne-hi.png", // url
        scaledSize: new google.maps.Size(50, 50), // scaled size
        origin: new google.maps.Point(0,0), // origin
        anchor: new google.maps.Point(0, 0), // anchor
    };
    var icon2 = {
        url: "https://cdn-icons-png.flaticon.com/512/1119/1119071.png", // url
        scaledSize: new google.maps.Size(50, 50), // scaled size
        origin: new google.maps.Point(0,0), // origin
        anchor: new google.maps.Point(0, 0), // anchor
    };
    var center = null;
    var map = null;
    var currentPopup;
    var bounds = new google.maps.LatLngBounds();
    function addMarker(icon, lat, lng, info) {
        var pt = new google.maps.LatLng(lat, lng);
        bounds.extend(pt);
        var marker = new google.maps.Marker({
            position: pt,
            icon: icon,
            map: map
        });
        var popup = new google.maps.InfoWindow({
            content: info,
            maxWidth: 350
        });
        google.maps.event.addListener(marker, "click", function() {
            if (currentPopup != null) {
                currentPopup.close();
                currentPopup = null;
            }
            popup.open(map, marker);
            currentPopup = popup;
        });
        google.maps.event.addListener(popup, "closeclick", function() {
            map.panTo(center);
            currentPopup = null;
        });
    }      

    function initMap() {
        map = new google.maps.Map(document.getElementById("map"), {
            center: new google.maps.LatLng(40.8319300, 16.4521000),
            zoom: 16,
            mapTypeId: google.maps.MapTypeId.ROADMAP,
            mapTypeControl: true,
            mapTypeControlOptions: {
                style: google.maps.MapTypeControlStyle.HORIZONTAL_BAR
            },
            navigationControl: false,
            navigationControlOptions: {
                style: google.maps.NavigationControlStyle.ZOOM_PAN
            }
        });
        addMarker(icon, 43.8219355, 16.4221334, '<b>test1</b>');
        addMarker(icon, 43.8312300, 16.4121000, '<b>test2</b>');
        addMarker(icon2, 43.8319300, 16.4521000, '<b>test3</b>');
   }
</script>
    <body onLoad="initMap()" style="margin:0px; border:0px; padding:0px;">
       <div id="map"></div>
    </body>

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