It might be a bug in lemmy because the url and thumbnail_url fields in https://lemmy.world/api/v3/post?id=1759116 contain different URLs for the same image. My custom lemmy client shows both fields and all posts with images have two separate image URLs, which I bet is using twice as much storage on instances for all those duplicate images.
I assume it's because you also added an image to the post? I agree it's not exactly intuitive, but people tend to post either an image or a URL to a page, not a combination of any of those.