Unable to create image style preview if image field is using private file upload

[chrislaick]

I'm having issues with saving a media entity with Drupal core's image field. The image field is set to upload to private files. The image is uploaded into the field and a preview is shown, however the preview is a smaller rendered size of the original image - not a true image style copy. Inspecting the image, it's the same file size and intrinsic size as the original image.

Most importantly, on clicking Save on the media entity, I receive an error message and the media form is emptied. I have already set correct paths for public, private, and tmp directories in settings.php. Default download method is Public local files served by the webserver in /admin/config/media/file-system.

Error message
The form has become outdated. Press the back button, copy any unsaved work in the form, and then reload the page.