I just tested this on WP 4.6 and ACF 5.4.4 and it’s working as expected. I don’t know if there was a bug in the version you were using, but there is nothing mentioned in the readme that a bug related to this has been fixed.
Do you have acf_form_head(); in the template/theme before get_header();
If that’s not it then try deactivating plugins and maybe switching themes to figure out if there is some kind of a conflict.
acf_form_head() helps with the actual saving of the form. For me the issue is that the post_title input field is displayed on the page without a name attribute set, so the data isn’t submitted with the form.