Apple has changed the cropping tool so that, by default, it preserves the photo’s aspect ratio. This seems like a reasonable change, except when the reason you are cropping is to change a photo’s shape. In that situation, after you tap the crop tool, tap Reset (at the top of the screen). This will allow you to freely adjust the crop region.

For another photos-related change in iOS 14 (and one that is very puzzling) see iOS 14 Camera Setting Not Restored on Launch