In order to submit an order guide, a user needs to navigate through the pages 'Describe Needs', 'Choose Options', 'Checkout'. It is possible to navigate from 'Choose Options' to 'Describe Needs', then back to 'Choose Options'. Some data gets lost from the 'Choose Options' page when the user navigates from it to 'Describe Needs', and then back to 'Choose Options'.
The issue is not found on regular (UI16) pages, only in Service Portal.

1- Go to the /sp?id=sc_cat_item_guide&sys_id=6690750f4f7b4200086eeed18110c761 page.
2- Fill out the form and click Next.
3- Under "Lenovo - Carbon x1" check the box by 'Adobe Acrobat'.
4- Click the button Previous.
5- Click the button Next.
Notice the checkbox from step 3 is no longer checked.

This is expected behaviour and by design of the service portal navigation workflow.