The collapse field option has a minor bug within ACF Pro (v5.8.7). Here is how to reproduce it:
- Edit any field group
- select a field and edit the “Collapsed” option
- select any field inside this option
- delete the field you selected
- reset the “Collapsed” option back to “- Select -“
- save
- export JSON and search “collapsed”
The exported JSON file will still show the deleted field’s ID, and the button to collapse a group is still visible. Slightly annoying if you want to remove the collapse button.