1. do not define any markers or polylines inside data { } 

2. define variables inside setup { } and remove reactivity (do not use ref(var)). Example below.

    setup() {
        let marker = null;
        let gmap = null;
        let currentPolygon = null;
        return {
            marker, gmap, currentPolygon
        }
    },

problem with reactivity has been confirmed with

import {Loader} from "@googlemaps/js-api-loader";