I have found an easy solution for this issue and hope it may help. I also tested thoroughly. The fix is given below.

private var mgr:MarkerManager;

if(mgr==null)
{
mgr = new MarkerManager(map,{});
}else
{
map.clearOverlays()
mgr.clearMarkers();
map.clearOverlays()
}

NOTE: I have pasted below the complete code for viewing a sample example implemented with the fix for markermanager utitlity bug.

//*****************************************************

<![CDATA[
import mx.controls.Alert;
import com.google.maps.MapEvent;
import com.google.maps.Map;
import com.google.maps.overlays.Marker;
import com.google.maps.MapType;
import com.google.maps.LatLng;
import com.google.maps.LatLngBounds;
import com.google.maps.extras.markermanager.MarkerManager;

public var markerArr:Array = [];
private var mgr:MarkerManager;

private function createMarkers():void
{

var bounds:LatLngBounds = map.getLatLngBounds();
var southWest:LatLng = bounds.getSouthWest();
var northEast:LatLng = bounds.getNorthEast();
var lngSpan:Number = northEast.lng() – southWest.lng();
var latSpan:Number = northEast.lat() – southWest.lat();
markerArr=[]
for (var i:int = 0; i

//*****************************************************

/Pär