Hello,
I think I have found a bug which is easily reproducible on a clean project on the 4.16.2 version.
You put an actor directly in the map, and you add to this actor a child actor component. You set both of the actors to be replicated.
Now if you play directly in this map with a client, the replication of the child actor will work fine, but if you start on another map first, then you travel using GetWorld()->ServerTravel(), the ChildActorComponent will replicates, but not the ChildActor which will be null on the client.
As a workaround, instead of placing the parent actor directly in the map, I spawn it at the BeginPlay event, and then it works as intended.