Create blank project Third Person blueprint based.
Select ThirdPersonCharacter instance in the scene and click edit blueprint.
Add ChildActor component.
Set Child Actor Class in blueprint (any class AmbientSound for example). Compile and save blueprint.
Select ThirdPersonCharacter instance in the scene and modify Child Actor Class of the created component. For example select SpotLight.
Copy ThirdPersonCharacter and paste. The value of the Child Actor Class for copied actor is wrong (old value = AmbientSound).
It can be clipboard copy/paste issue, just guessing. I don’t know, you are on Windows, Mac, what machine. Sometimes copying/paste large data need some time. You should check that it was really copied. On my side Ctrl/c not always works. It can be UE issue too of course - sometimes child elements do not inherit changes, I posted different bug like this weeks ago, I think it was fixed, but can be more.