Pretty devastated. Haven’t worked on my site since last ACF update, whereupon I now find this same problem. The flexible field in question was established ages ago and has worked continually, perfectly, until now. Today, it dropped all but three of its fields. I tried setting up the fields again as per a backup on another computer (not updated to the recent ACF), but the fields refused to save. A lot of archival information has disappeared – this is very grave for me.
I don’t understand the solution proffered here as to whether the field is registered or not in functions.php. In my case, the ACF flexible field is set up in a custom post type registered in functions.php – is that what you mean??
This doesn’t explain why it was working before, but not now?
