Summary:
There is no smart value that returns the default value selected for a custom field on the. Settings > Issues > Custom Fields page. There might be some scenarios where the customer may want to reset the field value via automation with the default value set in the custom fields page.

Workaround:

Hard code the automation rule to set the default value of your choice. The project admin needs to make sure that the default value in the automation rule is always the same as selected on the customfields page.

Note:
The earlier smart value 'issue.[Custom Field].defaultValue' is not supposed to work.