Custom field default values aren't applied to optional field in email submission

When an issue is submitted by email to the Service Desk mail handler, the default value is not applied to the issue for select lists if it is marked as optional.

Steps to reproduce:

1. Create a custom field called "contact type" that is a select list
2. Add values of "helpdesk", "email", and "other"
3. Set default value to be "email"
4. Make the field visible in Service Desk, and mark as optional
5. Submit request by email
6. View issue and note that custom field is not present on issue in JIRA view

Expected results:
The value for "contact type" is email.

Actual results
It seems to ignore the field completely, and is not visible on the Issue View in JIRA