With 100.5, the only thing printed is "Opacity changed". Looks like this is an issue with all the ImageAdjustmentLayers.
Are the others supposed to fire too?
var layer = new ArcGISTiledLayer();
layer.PropertyChanged += (s, e) =>
Debug.Print(e.PropertyName + " changed");
layer.Opacity = 0.5;
layer.Brightness = 50.0;
layer.Gamma = 50.0;
layer.Contrast = 30.0;