I am having the same issue
The author drop down only shows a user that is set to an administrator role.
Changing an user role to Contributor, Editor or Author removes that user from the author drop down and they can no longer be selected
ONLY a user assigned an administrator role displays.
This has only occurred since the upgrade to 5.9, rolling back to version 5.8.3 shows all users assigned as Admin, Editor, Contributor and author as expected in the drop down
