Sorry, I just realised the pasteboard links in my original message don’t play well with this forum’s interface. I have attached them again.
After much testing we discovered the issue was being caused by Cloudflare – https://www.stevejenkins.com/blog/2012/05/fixing-wordpress-admin-issues-with-cloudflare-page-rules/
We setup page rules to exclude wp-admin from various Cloudflare components and this issue resolved. Hopefully this post will save others some time.
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!