Missing data in commerce_registration_information_review

I am experiencing an issue where checkout is unable to complete on certain registrations because of missing data within the function commerce_registration_information_review.

It seems that in certain circumstances the $order->data array does not have a valid value for the iterator $i in the line:

$entity = $order->data['register_entities'][$prodkey][$i];

It also seems that sometimes the following line results in a EntityMetadataWrapper Exception because this function call returns FALSE:

$email = $reg_wrapper->mail->value();

This line was introduced in this patch: https://www.drupal.org/project/commerce_registration/issues/1901854#comm... →

I've been unable to determine exactly why certain registrations fail at this stage. Many work perfectly, and I've seen 3 registrations going through a checkout process where 2 of them work as designed and the third fails. From my testing this seems to only occur when multiple registrations are purchased in a single order. It affects both anonymous and logged in users.

The Entity module was updated to 7.x-1.9 in February: https://www.drupal.org/sa-contrib-2018-013 →

This site is using the 1.x branch of Entity Registration: https://www.drupal.org/project/registration/releases/7.x-1.6 →

What could be causing index lookup failures and empty email field data? Any advice or troubleshooting tips would be appreciated.