This seems to be related to the fix for [Link Removed]. Part of that fix made sure that the child actor component recreates the child actor when GIsReconstructingBlueprintInstances is true, as previously, the editor's client instance could be missing the child actor.
However, it seems that not only is the client still recreating the child actor on startup, but it is now also spawning another instance of the child actor when receiving the first bunch from the server for this actor. This is causing a duplicate instance of the child actor existing on the server.
Place a replicated actor with a replicated child actor component/child actor into a world partition map.
Start a multiplayer PIE session, and observe that the parent actor has two instances of the child actor on the client.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-200690 in the post.