I have made two automation rules in Jira;
Rule 1: To auto-populate a custom field (which is muti select field) in backlog to all the subtasks under that backlog
Rule 2 : To auto-populate the custom field in the backlog to all linked issues in that backlog which has issue type = bug.
Issue : A bug might be linked with multiple backlogs, and all the values of that custom field in all the associated backlogs needs to be populated in the bug and their subtasks.
Add to existing values is not useful in this scenario.