the icon is passed in by yourself. You can use it normally if you don't use the default icon.

(function(){
     var myIcon = L.icon({
         iconUrl: marker_icon,
         iconSize: [20, 20],
         iconAnchor: [10, 20],
    });

    var map = L.map('map-osm-leaflet').setView([51.505, -0.09], 13);

    L.tileLayer('https://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png', {
        attribution: ' <a href="https://www.openstreetmap.org/copyright">OpenStreetMap</a> contributors'
    }).addTo(map);

    L.marker([51.5, -0.09],{
        icon: myIcon
    }).addTo(map)
        
})();

that's the problem for me, too. The default icon in my project can't use the same error. I don't know why it becomes base64, in my project, but it becomes cdn, on the official website

the later solution is to manually introduce leaflet icons

import icon from "leaflet/dist/images/marker-icon.png";
import iconShadow from "leaflet/dist/images/marker-shadow.png";

let DefaultIcon = L.icon({
            iconUrl: icon,
            shadowUrl: iconShadow
        });
L.Marker.prototype.options.icon = DefaultIcon;
new L.marker(//marker).addTo(this.map);