Losing metadata when uploading an image in Django Admin

Question

I want to add a copyright notice into exif data of my displayed images.

I'm using django-imagekit and piexif libraries.

I can't manage to preserve nor enrich metadata of an image when I upload then process them in my Django admin.

Here is my model :

class Still(models.Model):
    image = ProcessedImageField(upload_to='images',
                                           processors=[ResizeToFit(1024)],
                                           format='JPEG',
                                           options={'subsampling': 0, 'optimize': True, 'quality': 85}
                                           )

    img_th_watermarked = ImageSpecField(source='image',
                                    processors=[ResizeToFit(width=612), TextOverlayProcessor(), ExifCopyrightProcessor()],
                                    )

Here is the processor :

def add_copyright_exif(image):

    copyright = "© {} my name".format(datetime.now().year)
    try:
        exif_dict = piexif.load(image.info["exif"])
    except KeyError as k:
        exif_dict = {"0th":{}}

    exif_dict["0th"][piexif.ImageIFD.Copyright] = copyright
    exif_bytes = piexif.dump(exif_dict)
    new_image = image.copy()
    new_image.info["exif"] = exif_bytes
    print(new_image.info)
    return new_image

class ExifCopyrightProcessor(object):
    def process(self, image):
        return add_copyright_exif(image)

The print seems to display the correct info, other processors work fine, but metadata get lost on the way. I tried removing format option as well. I don't mind that metadata get lost at the ProcessedImageField step, but I should be able to add new info at the ImageSpecField step.

Images and cached versions are stored on S3.

Thanks in advance for your help.