Session data partialy deleting

During the checkout process in confirm.php we are loosing session data in the order:
$order_data['customer_group_id'] = $this->session->data['guest']
$order_data['email'] = $this->session->data['guest']['email'];
$order_data['telephone'] = $this->session->data['guest']['telephone'];
$order_data['custom_field'] = $this->session->data['guest']['custom_field'];

First name and Last name are not lost in the same orders.
Payment method and Shipping method do not effect the issue.
The issue is happening in 6 orders out of 500 orders in a period of 3 days.
Opencart version: 3.0.3.2
One page checkout module.

Does anyone have similar problems with session data missing partially?

Since you're using a third-party checkout extension, this issue is likely related to the extension. You should try contacting the developers of the extension for support instead.