That’s sure weird. I’ve just tested it on my installation but failed to reproduce the issue.
Have you tried to reproduce the issue with WordPress’ stock theme (such as Twenty Sixteen) activated?
If the issue persists, could you please open a new ticket and provide temporary admin credentials to your site? You can open a new ticket here: https://support.advancedcustomfields.com/new-ticket. Also, please don’t forget to explain the issue again and provide the link to this thread.
Welcome to the Advanced Custom Fields community forum.
Browse through ideas, snippets of code, questions and answers between fellow ACF users