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.

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.

In testing on Xcode 12.0 beta 6, in the iOS 14 Simulator, when attempting to import a QuickTime movie from PHPicker using itemProvider.loadFileRepresentation, I got the following error: "Cannot load representation of type com.apple.quicktime-movie". I was able to workaround this by using PHPickerConfigurationAssetRepresentationModeCurrent. Also in testing, I found that itemProvider.loadFileRepresentation does not work for live photos. I had to use itemProvider.loadInPlaceFileRepresentation instead. Hope this helps!