I’ve checked your site and see you’re using custom table to store custom fields. However, I can’t find the code that create the custom table. Maybe you missed it, and that’s why the custom fields are not saved. Can you please check that again?
I’ve put the requests for group in my task list. Our team will work on that as soon as we can.
Update: We have resolved the issue. The problem is that the meta box uses a custom table but there’s no custom table registered. Deselect the checkbox to let meta box saves to custom table fixes the bug.