Why is the URI for updating the caption of an Instagram post not actually working, even though I receive a success message?

I am using Facebook graph API Facebook graph API, to update my Instagram page's post create/ update. the endpoint I am using is like

https://graph.facebook.com/{media_id}?caption={new_caption}&access_token={access_token}

Neither using insomnia nor using JAVA code do I receive the successful message, and the caption remains unchanged.

    private static CompletableFuture<PostFB> updatePostInstagramMessageAsync(String hostname, PostFB ig, HttpClient client) {
    JSONObject jsonPayload = new JSONObject();

    if (ig.text != null && !ig.text.isEmpty()) {
        jsonPayload.put("caption", ig.text);  
        jsonPayload.put("comment_enabled", true);  

        System.out.println("jsonPayload is " + jsonPayload.toString());

        String url = "https://graph.facebook.com/v19.0/" + "my postID";
        HttpRequest request = HttpRequest.newBuilder()
                .uri(URI.create(url + "?access_token=" + ig.page_access_token))
                .header("Content-Type", "application/json")
                .method("POST", BodyPublishers.ofString(jsonPayload.toString(), StandardCharsets.UTF_8))
                .build();

        return client.sendAsync(request, HttpResponse.BodyHandlers.ofString())
                .thenApply(response -> {
                    System.out.println("Response for updating post is " + response.body());
                    if (response.statusCode() == 200) {
                         
                            System.out.println("Updated post successfully.");
                        } else {
                            System.out.println("Failed to update post: " + response.body());
                           
                        }
                    } else {
                        System.out.println("Failed to update post. Status code: " + response.statusCode());
                       
                    }
                    return ig;
                })
                .exceptionally(ex -> {
                    ex.printStackTrace();
                   
                    return ig;
                });
    } else {
        return CompletableFuture.completedFuture(null);
    }
}

Despite receiving a successful response, the caption does not change. I have created the application and assigned the necessary permissions:

• pages_show_list,
• instagram_basic,
• instagram_manage_comments,
• instagram_manage_insights,
• instagram_content_publish,
• instagram_manage_messages,
• pages_read_engagement,
• instagram_manage_events,
• public_profile

I am able to create new posts but I cannot modify them. Why? any help would be appriciated.

https://developers.facebook.com/docs/instagram-api/reference/ig-media#updating

Updating
POST /{ig-media-id}

Enable or disable comments on an IG Media.

What that last line says, is the only thing you can do via this update endpoint.

You get a success response, because you successfully set or changed the value of comment_enabled.

The additional parameter caption you are passing, has no meaning for this endpoint, and simply gets ignored.