C# WPF - Creating Png-Image File of Usercontrol results in black image

Question

I have a Stackpanel inside a Scrollviewer where Canvas elements are inserted in the code behind.

Each Canvas Element has a Image, and some Labels, that can be moved around inside the Canvas. For each Label inside each Canvas i want to create a Png Image. My goal is to insert those Png Images to a existing pdf file later. The problem is that i only get black images.

I have already searched with google and stackoverflow but did not really find an answer.

The labels are added to the Canvas by Code and are not created in xaml. I also tried to create a Png for each Canvas instead of the labels. The image for the first Canvas was created correctly. But the second was black again.

I would be really thankful if someone knows what i am doing wrong.

foreach (Canvas canvas in StackPanel_Canvas_Container.Children)
            {
                foreach (object o in canvas.Children)
                {
                    if (o.GetType().Equals(typeof(Label)))
                    {
                        Label label = (Label)o;
                        label.Background = Brushes.Transparent;
                        label.Foreground = Brushes.Black;
                        label.UpdateLayout();
                        RenderTargetBitmap bitmap = new RenderTargetBitmap((int)label.ActualWidth, (int)label.ActualHeight, 96, 96, PixelFormats.Pbgra32);
                        bitmap.Render(label);
                        String path = System.IO.Path.Combine(Directory.GetCurrentDirectory(), "created_images");
                        if (!Directory.Exists(path))
                        {
                            Directory.CreateDirectory(path);
                        }
                        String tmp = string.Empty;
                        do
                        {
                            tmp = System.IO.Path.Combine(path, $"img_{++i}.png");
                        } while (File.Exists(tmp));


                        using (FileStream stream = File.Create(tmp))
                        {
                            PngBitmapEncoder encoder = new PngBitmapEncoder();
                            encoder.Frames.Add(BitmapFrame.Create(bitmap));
                            encoder.Save(stream);
                        }
                    }
                }
            }