There isn’t really any way to tell from what you’ve supplied. It could really be many things.
I would start with the basic WP debugging steps. Deactivation of other plugins and changing theme to see if you can figure out is something else is causing it.
My first thought when you say “migrating to ACF” is that you have other fields with the same names as the ACF fields, this could cause the problem.
Another issue could be that you are exceeding the php setting max_input_vars on your server.
All of these are just guesses.
