Data structures for field groups created via UI are not visible in $form via hook_form_alter

I have a problem where newly created groups that have been created via the UI are not accessible via hook_form_alter.

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I've a content type where fields have been arranged (for node editing) using a horizontal tabs fieldset. I need to create two new tabs programmatically and insert a views block in each.

I've attempted to create these extra tabs using hook_form_alter - but the parent group (created via the UI) isn't visible in the $form array. Modifications can successfully be made to the standard 'additional_settings' group, as this is visible.

I've tried to alter the weight of my custom module so it fires after field_group and I've also tried to use hook_form_FORMID_alter(), and I've also tried to use $form['#after_build'][] to add a callback function that's run after the form has been built but none of these solutions have allowed me to modify the field group fieldsets/groups.

Any advice greatly appreciated.

Luke