What I’ve tried: Disabling all other plugins, changing theme to 2019.
I’m creating a group field for an options page. After creating the group, I add subfields. I save the page, and the subfields are saved as regular fields instead of being nested within the group field.
Hope someone has some insight into this, because I can’t find any post with this exact issue. Some report that similar issues are caused by plugin conflicts, or not naming the parent group, but neither of those issues exist here.