Classic Editor Author/Instructor Metabox Saving issue

### Reproduction Steps

With Classic Editor turned on, create a new course.

You will be able to set the first instructor in the instructor meta box and save.

This first instructor will become the author of the course.

Once this first save is done, edit the course and change the first instructor in the instructor meta box then save.

The instructor will change, but the author meta box will still have the original author/instructor saved and will not find users in the role instructor, only admins.

What should happen is the first instructor in the instructor metabox should be saved to the author metabox.