I JUST had this issue and I just spend the last 30 minutes troubleshooting it. For some reason #acf-field-group-fields no longer had the .acf-postbox class and only had .closed class on mine. I removed the .closed class and added the .acf-postbox then saved the field group and it came back.
I’m not sure if this is the same for everyone who has had this issue but I hope it helps!
