Looking at the copyright at the bottom iof the map it suggests you are using the Leaflet API. This API allows the Anchor point to be specified...
var myIcon = L.icon({
iconUrl: 'my-icon.png',
iconSize: [38, 95],
iconAnchor: [22, 94],
popupAnchor: [-3, -76],
shadowUrl: 'my-icon-shadow.png',
shadowSize: [68, 95],
shadowAnchor: [22, 94]
});
If SP Page Builder does not provide a way to adjust the anchor point then it seems it is NOT a problem of OpenStreet Map.
Please look at the map again as I have placed a default marker at the exact same location as the custom marker. When you zoom out you can see where the anchor point is of both the default and the custom marker and you can see that the anchor of the custom marker needs to be adjusted to function properly.