If you want a free alternative that has more functionality but isn't quite as simple consider GIMP (see how to remove image backgrounds in GIMP) or.
Paint is tantalizingly close to being usable as a basic photo editor, but it still lacks some key features such as a magic wand selection tool, the ability to draw selections by aspect ratio, filters for blurring, and drop shadows. So it's interesting to see Microsoft actually adding features to this program. At one point, in 2017, Microsoft was going to remove Paint from the OS and replace it with Paint 3D, but the tide turned in 2019 and Paint 3D is now no longer a default, while Paint remains. Microsoft Paint debuted in Windows 1.0, which came out in 1985, so this app is nearly 40 years old.
Unfortunately, since Paint doesn't have a magic wand tool, there's no way (within the app) to remove the white box. The issue with the white box appeared on several other photos I tried to remove backgrounds from, so it's not a fluke.
