No it is something stranger. And not the first time:
I have this got earlier that all the fields are removed from the Custom Field after that I moved them up and down. After hit save all is removed, only the Field Group page exist without any fields. I have some images of that.
I add the first two fields and save them.
Another day I add two extra fields and save the again.
After that I move the fields (the last two to the beginning).
When I hit save the total Custom fields are empty.