I discovered that IE has an issue displaying SVG google maps markers, as discussed here:
I was hoping you would be willing to add support for this in an upcoming version of the Cornerstone plugin. For now I have edited plugins/cornerstone/assets/dist/js/site/cs-body.js to include this modified line:
new window.google.maps.Marker({map:e,position:new window.google.maps.LatLng(a.lat,a.lng),infoWindowIndex:o,icon:{url: a.image, scaledSize: new google.maps.Size(90,90)},optimized:false});