Custom thumbnail upload succeeds but is not applied to YouTube Shorts

Question

I’m uploading a custom thumbnail for a YouTube Shorts video using the YouTube Data API v3. The API response indicates success, but the uploaded thumbnail is not applied to the video.

if thumbnail_path:
            try:
                # Verify thumbnail meets requirements
                img = Image.open(thumbnail_path)
                logging.info(f"Verifying thumbnail dimensions: {img.size}")
                if img.size != (720, 1280):
                    logging.error(f"Invalid thumbnail dimensions: {img.size}. Expected: (720, 1280)")
                    img.close()
                    return video_id
                img.close()

                if os.path.getsize(thumbnail_path) > 2 * 1024 * 1024:
                    logging.error("Thumbnail file size exceeds 2MB limit")
                    return video_id

                # Create MediaFileUpload object for thumbnail
                thumbnail_media = MediaFileUpload(
                    thumbnail_path,
                    mimetype='image/jpeg',
                    resumable=False
                )

                # Set thumbnail with proper request
                thumbnail_response = self.youtube.thumbnails().set(
                    videoId=video_id,
                    media_body=thumbnail_media
                ).execute()

                if 'items' in thumbnail_response:
                    logging.info(f"Thumbnail successfully set for video {video_id}")
                    logging.info(f"Original thumbnail dimensions: 720x1280")
                    logging.info(f"YouTube generated thumbnails: {thumbnail_response['items']}")
                else:
                    logging.warning(f"Thumbnail upload response missing items: {thumbnail_response}")
                
                # Wait for thumbnail processing
                time.sleep(10)  # Increased wait time
                
            except Exception as e:
                logging.error(f"Failed to upload thumbnail: {str(e)}")
                logging.error(f"Thumbnail path: {thumbnail_path}")
                logging.error(f"Video ID: {video_id}")
                logging.error(f"Full error: {traceback.format_exc()}")

The uploaded thumbnail meets all the documented requirements:

1. Dimensions: 1280x720
2. File size: Less than 2MB
3. Format: JPEG

Is this a limitation of YouTube Shorts, or is there something specific I need to do to apply a custom thumbnail to Shorts?