opened_by field was added to the sn_customerservice_case form. When editing existing cases or creating new cases, the field is displayed but after some seconds it disappears.
For the specific case of the sn_customerservice_case table, the following Out of the Box client script is the one causing this behavior:
Name: Show partner field
Description: Shows "opened_by" on the customer creation form, if the account for the case creator is partner of someone
Evaluate if the opened_by field should be displayed even when the account for the case creator is not partner of someone, in that case inactivate the client script.