It does seem that when I create the Field Group and save the checkbox default settings, the settings are indeed saved. The options and defaults are shown on the page editor as expected. It seems the defaults are not properly loading in the ACF Field Editor and therefore appears empty. If you save any changes to the Field Group, the defaults are lost.
Hope that helps in finding the issue.
Welcome to the Advanced Custom Fields community forum.
Browse through ideas, snippets of code, questions and answers between fellow ACF users