Until this issue is resolved you could workaround this issue like follows:
- use the loaded event of the window to get the TemplatePart holding the Image -> "PART_Icon"
- set its width and height to the default size 16x16.

void MainWindow_Loaded(object sender, RoutedEventArgs e)
        {
            var image = this.ChildrenOfType<Image>().FirstOrDefault(x => x.Name == "PART_Icon");

            if (image != null)
            {
                image.Width = 16;
                image.Height = 16;
            }
        }