"This registration is for" and related fields are empty with use of "Go Back" button on page after Registration Information page

it looks like this is because the registration entity's registration_id is not set

It looks like the referenced user comes back empty as well. This is all related to the fact that the Registration object is new and doesn't have an id yet, or a referenced user object:

from registration_form():

//who_is_registering
'#default_value' => (isset($registration->registration_id) ? $registration->registrant_type($user) : NULL),

//user
'#default_value' => ($registration->user ? $registration->user->name : ''),

These lines of code in the registration module fail in the use case of commerce_registration. The registration module set these functions up not expected a "new" Registration entity to be loaded into the form.

Patch imminent

patch