When called via <maplink>, markers stored in a Commons .map page will display a broken image icon when the marker uses the "marker-symbol": "-letter" or "marker-symbol": "-number" properties. The markers display correctly on the .map page itself.
<mapframe width="300" height="300" latitude="-34.859" longitude="138.465"> { "type": "ExternalData", "service": "page", "title": "Sandbox/Gareth/Sandbox.map" }