Hopefully, there's an important clue about the Unknown fields error: In this automation, in sub-task create actions for a different workflow that occur after the one that produces the error, no such error occurs. Here are the relevant screen shots:
Set Assignee that produces the Unknown fields error in an action to create a sub-task

Unknown fields error

Assignee field is present but not populated, in the target sub-task issue view screen

Assignee field is present in the trigger issue view screen

Successful Set of Assignee field in action in create a sub-task in a different workflow
