I think you need to actually publish or save a post for it to remember the collapsed state of the fields, but I’m not sure. It could also be that if you’re field group is extremely complex with lots of nested flex fields and repeaters that it’s confusing ACF trying to figure out what fields should be collapsed and what ones shouldn’t be. It could be a bug, or it could just be that you’ve found a limitation. Field collapsing is a relatively new feature of ACF.
