Going forward with 4.20 you will only be able to set actor references on an instance of a Blueprint in a level, but not in the class defaults of a Blueprint.
This is a regression from 4.19. In 4.19 you were able to change the actor assigned to the array element in a Camera Actor variable. In 4.20 and 4.21 the field is grayed out and displays None with a tool-tip that says "Editing this value in a Class Default Object is not allowed."
Results: You can't change the actor assigned to the array element
Expected: You can change the actor assigned to the array element
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-60132 in the post.