Adding to this. Have the same issue, except I’m using the ACF UI instead of coding it.
It doesn’t seem to be related to plugins or themes (I’d disabled all of the non-essential plugins and themes).
It doesn’t produce any PHP errors oddly enough (or at least none that I could find with debugging and logging on).
It also doesn’t seem to be related to PHP variables (post max size and the such) as all of those are set pretty generously.
Genuinely curious as to what’s causing this.
