Epic Link default value in request type form doesn't reflected in ticket when ticket raised by portal only customer from portal

Issue Summary

When a request type form has "Epic Link" set to hidden and has a default value, it doesn't work when a ticket is raised by portal only customer account from the portal.
This is reproducible on Data Center: (yes) / (no)

Steps to Reproduce

1. Open request type form
2. Add "Epic Link" to the form
3. Set to hidden and set a default value
4. Log in to the customer portal using a portal-only customer account.
5. Raise a ticket from the portal
6. Check the ticket in JIRA

Expected Results

Epic link field updated with the default value.

Actual Results

The epic link field is empty

Workaround

Set the Epic link field value using automation.