I've created a custom field which is not required.
In the workflow I set required this field for some status. When I change status on an issue, I can see the red "star" (as required) next to this custom field, on the ticket site, however the system accepts the "submit" without fill out this field. This only happens if the field format is list and the multiple values is acitve. If the field is list field, but the multiple values is not active, I can not submit without fill the field.
So it's maybe a bug.