When a blueprint is already placed in the world and another component duplicate added to it in the BP asset (in my case as a child of a camera) then that attached like the component it was duplicated of and the rotation set back to relative zero. The rotation of that component of the actor already placed in the world will not have that zero rotation.
I was trying to repro this with the steps you gave and my rotation on the child camera does not zero out in 4.18. If you could provide more detailed repro steps so I can try again or attach a small project that would be great.
I am marking this as solved for reporting purposes. If you are able to provide files or need any more assistance with this please comment/respond here and it will reopen your question.
Yea, that is the bug. You set the relative rotation of the duplicated component to zero. Then you look in the actor already placed in the world. The new component will NOT have that zero relative rotation.
I misunderstood what you were trying to relay then. I think you are saying the blueprint you made is setting the relative rotation of the child camera but it does not set it to 0. If that is the case then I’m definitely going to need some detailed reproduction steps of what you were doing. Also when you play in editor does the rotation change?