When creating a group, if creator membership was enabled, you would be shown a second page where you could configure the membership data. This was often confusing as there would be very little or even nothing to show on said page. Similarly, the same was true for creating new content (nodes, subgroups, ...) in a group, if the plugin was configured to use the wizard.
Now, all fields (if any) are shown on the form you are mainly interested in: The group form or the content entity form. This happens via the CreateFormEnhancer service.
Warning
This means your form alters to group relationship forms now only apply to operations other than create. If you want to alter the relationship fields on the create form, you need to alter the group or content entity form instead. The CreateFormEnhancer has a helper method to detect if the form was enhanced: isFormEnhanced(array $form)
