The stamps look perfectly fine on my Samsung Tab S4. Can you take a screenshot of what it looks like on your device? I'm not sure why it's behaving oddly for you at the moment. Perhaps there is a setting that is different between our devices. Did you make sure that no color is selected for the stamp? If you have a color selected, the pixel blending could be causing some undesired effects because your images have a white background. Just tap the color field and select "No Color".
Mike
Mike