If I create the work item manually, both assignee and approver fields are on the create screen and I can set their values.
My automation rule is triggered on schedule with a JQL. For each matching work item, I create a new work item and copy some fields over.

I tried using the COPY option for Assignee and Approvers at first. Summary and Description are copied, but Assignee and Approvers remain blank.
Then I tried setting a fixed value as above. Still blank.
No error in the audit log, it simply lists the work items created.