Welcome to Microsoft Q&A!
How do you generate the icons for your app? Did you just generate all the icons using the Visual Studio tool?
This behavior is caused by the icon you are using for your app in the App manifest file. When you check the icons in the appxmanifest -> Visual Assets -> App Icon, you could see that the icons are not fully filled with the image you choose. If you want to show a fully filled image, you will need to generate the icons yourself.
For more information, please refer to App icons and logos to check how the icons are applied to the system.