There are things that can make fields disappear, but, that usually happens when you have many, many fields and is associated with a PHP setting. From what I can see you don’t have nearly enough fields in the group to cause this.
What’s really odd is that it appears the fields are all there, because the field order number is not in sequence and it’s like some of the fields are hidden. When you get this happening try inspecting the source of the page and see if you can find a reason that some of the fields may be hidden.
You could try turning on error reporting and the next time it happens look in your error log to see if any PHP errors occurred. But you’re right, unless you can give me a way to reproduce the error it will be extremely hard to find. 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!
The ACF site is currently experiencing some issues.— Advanced Custom Fields (@wp_acf) September 21, 2022
We apologize for the inconvenience and we are working hard to get it back up and running as soon as possible.
© 2022 Advanced Custom Fields.