@bronson That solved it for me too. Thank you! This happened to me using the most recent version of ACF in 2021. Don’t know what causes this weird hiccup.
In case this helps someone else: it was happening to me only on certain field groups inside a flexible content field group. But you have to duplicate the whole flex content group and delete the old one. It won’t work if you just delete the fields inside the flex content field.
