Update: In testing with Xcode 12.0.1, running on a physical device with iOS 14.0.1, loadPreviewImage also returns nil in the completion handler.

Update: For now, I have decided to go with using QLThumbnailGenerator - https://developer.apple.com/documentation/quicklookthumbnailing/qlthumbnailgenerator to create thumbnails for resources imported from PHPicker. In the future, this generator should be very easy to swap out for NSItemProvider method loadPreviewImage if that starts working on PHPicker.