Found this issue because I faced the same problem on a site now, with ACF5 Pro. If you’re finding this thread searching around the solution is the same except in ACF5 as you can see below the priority on admin_menu is 99.
From pro\admin\options-page.php constructor:
add_action('admin_menu', array($this,'admin_menu'), 99, 0);
So the solution is use priority 100 or more in your admin_menu call as shown below:
add_action( 'admin_menu', array( $this, 'adminMenuPages' ), 101);
