Thanks for getting back to us.
We have tested the scenario by using a sample image for
ImageStamp as watermark with latest version of the API and we were unable to notice the issue which you have shown in the shared screenshot. Please check following code snippet for the image stamp properties which we used to add it inside your PDF. A sample output PDF is also attached for your kind reference.
imageStamp.VerticalAlignment = VerticalAlignment.Center;
imageStamp.HorizontalAlignment = HorizontalAlignment.Center;
imageStamp.Opacity = 0.5;
imageStamp.RotateAngle = 45;
WatermarkTest_out.pdf (106.9 KB)
Would you please share the same image which you have used for the watermark. We will again test the scenario with your image and let you know about our findings. We also request you to please try your scenario using Aspose.PDF for .NET 18.7 and share with us if issue still persists.