I’m not sure of all of the details of the title_save_pre hook. If it is firing for acf_form() saves then most likely the global $post has no value. You will also not have a way to get the post or post ID in this function since the value of the $_POST post ID will likely be new_post.
It is always best to use the acf/save_post hook when dealing with ACF, front end or back https://www.advancedcustomfields.com/resources/acf-save_post/
You could also create your own acf/pre_save_post for the front end https://www.advancedcustomfields.com/resources/acf-pre_save_post/
