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When editing a Custom Field Group and choosing on which post edit screens the field group should be shown, one option is to specify a Content Template, such that any post which uses the template will offer the custom field group fields.
The “default” option (referring to posts which have no template assigned) does not work. When you check the option and save the field group, the option does not remain checked (and hence does not work).
Until this is fixed you’ll need to find an alternative rule for determining where to display the custom field group.
My original post regarding this issue was marked as resolved, but presumably it’s still in the developer queue. For reference, this is the original reported problem:
This has been an issue for as long as I can remember. I am trying to assign a custom field group to the ‘default’ page template only. However, if I check the ‘Default’ checkbox under Edit Post Field Group / Templates, after saving it does not keep the box ticked and says “By default this group of fields will appear when editing all content.” The group then appears on every single page, post and CPT.
All other checkboxes retain their ticked status. This is not specific to a single site – it has been the case for every website and theme I’ve tried it on in the past.