A demo WordPress plugin of how you could disable

acf-blocks-v3-disable-preview.php

<?php
/**
 * Plugin Name: ACF Blocks v3 Disable Preview
 * Plugin URI: https://liamg.dev
 * Description: Disables the preview/iframe rendering for ACF Blocks v3 that have mode support disabled and edit mode set.
 * Version: 1.0.0
 * Author: Liam Gladdy
 * Author URI: https://liamg.dev
 * License: GPL v2 or later
 * License URI: https://www.gnu.org/licenses/gpl-2.0.html
 * Text Domain: acf-blocks-v3-disable-preview
 */

// Exit if accessed directly.
if ( ! defined( 'ABSPATH' ) ) {
	exit;
}

/**
 * Intercept ACF block registration to replace render callback for v3 blocks with mode disabled.
 * Runs AFTER ACF's own filter at priority 99.
 *
 * @param array $settings The block settings after ACF processing.
 * @param array $metadata The raw block.json metadata.
 * @return array Modified settings.
 */
function acfbv3dp_replace_render_callback( $settings, $metadata ) {
	// Only process ACF blocks
	if ( ! isset( $metadata['acf'] ) || ! $metadata['acf'] ) {
		return $settings;
	}

	// Check if this is a v3 block with mode support disabled
	$is_v3         = isset( $metadata['acf']['blockVersion'] ) && $metadata['acf']['blockVersion'] >= 3;
	$mode_disabled = isset( $settings['supports']['mode'] ) && $settings['supports']['mode'] === false;
	$has_edit_mode = isset( $metadata['acf']['mode'] ) && $metadata['acf']['mode'] === 'edit';

	// If this is a v3 block with mode disabled and edit mode set, replace the callback
	if ( $is_v3 && $mode_disabled && $has_edit_mode ) {
		// ACF sets this to 'acf_render_block_callback' at line 160 of blocks.php
		// We override it here to use our custom callback instead
		$settings['render_callback'] = 'acfbv3dp_no_preview_callback';
	}
	return $settings;
}
add_filter( 'block_type_metadata_settings', 'acfbv3dp_replace_render_callback', 98, 2 );

/**
 * Custom render callback for blocks that don't support preview mode.
 *
 * Returns ACF's special 'acf-block-preview-no-html' value in the editor to trigger
 * the NoBlockPreviewPlaceholder component with the "Edit Block" button.
 *
 * @param array    $block      The block settings and attributes.
 * @param string   $content    The block content.
 * @param bool     $is_preview Whether or not the block is being rendered for editing preview.
 * @param int      $post_id    The current post ID.
 * @param WP_Block $wp_block   The block instance.
 * @param array    $context    The block context.
 * @return string Special value for ACF or empty string.
 */
function acfbv3dp_no_preview_callback( $block, $content = '', $is_preview = false, $post_id = 0, $wp_block = null, $context = array() ) {

	// If we're in preview mode, return ACF's special value
	// This triggers the NoBlockPreviewPlaceholder React component with "Edit Block" button
	if ( $is_preview ) {
		echo 'acf-block-preview-no-html';
	} else {
		// On the frontend, call ACF's original render function
		if ( function_exists( 'acf_render_block_callback' ) ) {
			acf_render_block_callback( $block, $content, $is_preview, $post_id, $wp_block, $context );
		}
	}
}