Epic Link custom field silently fails to set value

In JIRA Service Desk, you can set a default value for unsupported fields added to a request type.
As per GHS-12742 this now does not work in 7.0.0.
However, another problem is that testing with JIRA 6.4.10 with Agile 6.7.11 and Service Desk 2.5.8, the following is observed.
Reproduce
Start JIRA 6.4.10 with Agile and Service Desk plugins
Create a Service Desk project
Use a field screen that has Epic Link on the create screen
Go to Project Administration for service desk project -> Request Types
Edit fields for the request type
Add a new field (Epic Link)
Observe that it is added as a hidden field and requesting to preset a value
Click Edit value
Set a default value (may need to first create an Epic in the project, and add Epic to Issue Types)
Unable to render embedded object: File (epiclink-64.png) not found.
Now, when you create an issue through the customer portal.
Create issue as an agent for that Request Type from Customer Portal (the user you used to make the admin settings above) and observe that in JIRA view, the Epic Link has been set as expected
Now, create a Service Desk customer. Best method to do this is to Invite a Customer from the Project -> People page. Then follow the email to set password (or go to User Management and set a password)
Log in as this new customer only user to the Customer Portal and create an issue for the Request Type
Log in as an agent again, so can view issue in JIRA Issue View
Expected outcome
Like when performing the action as an agent, the Epic Link is set correctly for the user
Or -> If this is is not allowed (and you should be a JIRA User to set this field), then ensure that a FieldValidationException is thrown when validated
Actual outcome
The setting passes field validation, and is contained in the Issue Update when saved into JIRA, but when viewing the issue later, that value has silently failed to be actually saved.
Workaround
No workaround, except removing Epic Link from the request type fields