Actors attached to a child actor component become unattached after viewport update

When an actor is added as a child to another actor's child actor through the world outliner, updating the editor viewport causes the added actor to become unattached from the child actor.

1. Open UE4 Editor (any project)
2. Add three blueprints based on Actor (Parent, Child, SubChild)
3. Open Parent Blueprint and add Child Actor Component
   • Set Child as the child actor
4. Add Parent to viewport
5. Add SubChild to viewport
6. In the world outliner, drag SubChild to Child
7. Click the dropdown arrow next to Build and select Build Geometry

Result:
SubChild becomes unattached from Child after build

Expected:
SubChild remains as a child.