I’ve been pondering this and there is nothing specific that I can think of that could cause this to happen.
The only thing that I can come up with is that there is a filter somewhere that’s interfering with the saving of the field group. I would start by deactivating plugins and switching theme to see if I could narrow down where the conflict might be.
Welcome to the Advanced Custom Fields community forum.
Browse through ideas, snippets of code, questions and answers between fellow ACF users