I'm running the following handler to process request to a custom post type's contents

add_action('wp_ajax_uipp_get_button_content', 'uipp_get_button_content');
add_action('wp_ajax_nopriv_uipp_get_button_content', 'uipp_get_button_content');

function uipp_get_button_content() {
    if (!isset($_POST['button_id']) || !is_numeric($_POST['button_id'])) {
        wp_send_json_error(['message' => 'Invalid button ID.']);
    }

    $button_id = intval($_POST['button_id']);
    $button_content = apply_filters('the_content', get_post_field('post_content', $button_id));

    if (!$button_content) {
        wp_send_json_error(['message' => 'Content not found.']);
    }

    wp_send_json_success(['content' => $button_content]);
}

And then attempting to print the resulting content on a page with:

fetch(ajax_object.ajax_url, {
    method: "POST",
    headers: {
        "Content-Type": "application/x-www-form-urlencoded",
    },
    body: new URLSearchParams({
        action: "uipp_get_button_content",
        button_id: button_id,
    }),
})
.then(response => response.json())
.then(data => {
    if (data.success) {
        revealContainer.innerHTML = data.content;
        console.log(data.content);
    } else {
        revealContainer.innerHTML = `<p>Error: ${data.message}</p>`;
    }
})
.catch(error => {
    revealContainer.innerHTML = "<p>Failed to load content.</p>";
});

Inspecting in the console (Network > Fetch/XHR), fetching was a success, I can see the content there. But on the frontend, it always prints "undefined". enter image description here

Nothing in the console in the way of errors - error_log and debug_log are both empty!

Where could I be going wrong?

In the handler, I even tried replacing the sent data with just a simple "Hello", thinking that there might be special characters from the post content's HTML, styles, scripts, that could be interfering or causing malformed code.

Replaced: wp_send_json_success(['content' => $button_content]); with: wp_send_json_success(['content' => "Hello"]);

Same issue, still undefined.