Peter,
I created the field group at least two weeks ago and did not make any changes to it. I was using the fields in the two custom post types to capture data, and they were included in a WS-Form form.
When I went in to modify the structure of two of the fields, ALL of the fields were gone. So, I don't believe that this issue involved any double-clicking of the Update button. From what I can see, all of the field-level details become disassociated from the field group itself.
As I mentioned, my site is still in development, therefore this particular issue was a minor inconvenience that is easily repaired. My real concern is the risk of this happening after my site has gone into production and how I would recover from it, specifically to restore or re-capture the data values for each affected post.
In any case, I will monitor the situation and advise of any issues.
Please close this ticket.
Thanks.