Update - This issue is fixed as of the 2/5/2020 update.
As of the 1/30/2020 update, the Teacher Emulation for Gradebooks is not working. When selecting a teacher, nothing happens and the page is not displaying the teacher's gradebook.
Programming is looking into this now.
Work-around - Teacher's gradebooks can be accessed by using the "User" page and the "Log in as" button on the Teacher's User Account.