Problem : I want to auto copy the custom field (scope category );
1. from backlog to all the subtaks created under that backlog.
2. All the bugs to have the scope category from the parent linked backlog

This rule works fine, if the scope category field is a non-mandatory field. After the issue creation the scope category field from backlog gets auto populated in the scope category.
But If I make this field as mandatory, then during subtask creation the scope category values from the parent backlog is not getting auto populated (need to manually select which I don't want my users to do) thus I am not able to create a subtask.

Please provide suggestion on how to achieve this use case