Epic Link set default value not working in JSD when ticket raised by customers

Steps to Reproduce:

1. Create a JIRA Agile + JIRA Service Desk project
2. Visit the JSD settings
3. On a request type add "Epic link" to it (it will be a hidden field)
4. Set the default value to any Epic on your project

Expected Results:

1. When customers raised tickets (via email or Customer Portal) of that request the field "Epic Link" should be populated with the default value set.

Actual Results:

1. When customers raised tickets (via email or Customer Portal), the value is not populated on the "Epic Link" field.
   The same does not happen if the one raising the ticket is an agent.

Add-on Version:
JIRA v6.5-OD-08
JIRA Service Desk v2.6.0-OD-06
JIRA Agile v6.7.11-D20150803