If the image you upload is really not square, then this is not a bug. If so, you can send a suggestion in Ideas category to accept non-square images as AP avatars.
OK by a square the application really wants you to open up photoshop take your photo and literally make it pixel perfect where height matches length and if it’s one pixel off it will reject it. is that really intended behavior? Because this seems like a handicap the developers put into it to inconvenience people because almost every application on earth accepts profile pictures that don’t have to be a square
A more complete solution would be to allow cropping on upload. Though as a content creator, it’s reasonable to expect that one knows how to create a 1:1 image if it’s required.