consider this code:

const element = new google.maps.marker.AdvancedMarkerElement();
element.position = new google.maps.LatLng ();
const position2 = element.position;
position2 instanceof google.maps.LatLng 

Problem ! last statement ie evaluated to false !

My solution, set a Position class:

I use this class;

class Position {

    lat;
    lng;
    set (latlngObj) {
        if (latlngObj instanceof google.maps.LatLng) {
            this.lat = latlngObj.lat();
            this.lng = latlngObj.lng();
        }
        else {
            this.lat = latlngObj.lat;
            this.lng = latlngObj.lng;
        }
     }
     get () {
        return newLatLng (this.lat, this.lng);
     }
}