Do you have a custom validation or code that modify the posted data? Could you please share the JSON (PRO version) or XML (free version) export file and the PHP file of the form and the custom code (if you use it)?
Also, could you please try to reproduce the issue on one of the WordPress’ stock themes (like Twenty Fifteen) with other plugins deactivated? If it disappears, then you can activate the theme and plugins one by one to see which one causes the issue.
You can also try to activate the debug mode to find any error message on the page. This page should give you more idea about it: https://codex.wordpress.org/Debugging_in_WordPress.
Welcome to the Advanced Custom Fields community forum.
Browse through ideas, snippets of code, questions and answers between fellow ACF users
Helping others is a great way to earn karma, gain badges and help ACF development!