Hi James,
Exact the same issue here. I’ve added 10 custom fields a while ago to this custom post type. I want to edit these and can’t do that, the panel with custom fields is simply gone.
https://www.dropbox.com/s/wa95wfojmj8zrb8/Screenshot%202016-10-06%2017.13.57.png?dl=0
When viewing the inspector the custom fields div is actually there, but has a “closed” class on it.
https://www.dropbox.com/s/0yscosvi5gvdd9v/Screenshot%202016-10-06%2017.15.58.png?dl=0
When I remove the “closed” class the fields show up as intended.
https://www.dropbox.com/s/s0dth0g7thxaa5p/Screenshot%202016-10-06%2017.16.57.png?dl=0
Any idea?