Other store apps have full icon without the black border. It might not a big issue, but it doesn't really looks right.

• startscreen.png - it has black border on the start screen app list (not in Windows 10 20H2).
• pinned-taskbar.png - it also has the black border when pinned on taskbar
• pinned-start.png - it looks fine when pinned on start screen
• 0yfRuh0TjI.png - startscreen app list icon is fixed on Windows 10 20H2. but not in the taskbar icon.

Docs of app icons & logos for UWP app:
https://docs.microsoft.com/en-us/windows/uwp/design/style/app-icons-and-logos