Unexpected results from setting relative rotation on child of child in BP.
Also confirmed in Main CL 4302610
"I have found that changing this code from RelativeRotationCache . RotatorToQuat (NewRelativeRotation); to RelativeRotationCache.QuatToRotator(NewRotationQuat); in bool USceneComponent::InternalSetWorldLocationAndRotation(FVector NewLocation, const FQuat& RotationQuat, bool bNoPhysics, ETeleportType Teleport) in SceneComponent.cpp fixes the problem but there is a comment above that line explaining why the Quaternion is generated from the Rotator and not the opposite. I guess it could be fixed by checking if we are in the construction script (or if the component has not begun play yet) and generate the Quat from the Rot in that case, otherwise generate the Rot from the Quat. That still wouldn't fix the problem in BP and begin play but it is an improvements."
Example Project Attached
Result: Different rotation values will be set
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-63188 in the post.