Replicated Actors are first spawned on the Server and then Replicated/Spawned on the remote clients. The Server can’t send the RPC (Multicast) before the Client has spawned the Actor. This is why nothing happens when you call the RPC on BeginPlay.
Solution 1
Make a replicated variable set to “Instance editable” and “Expose on Spawn” and have the Server use SpawnActorOfClass and input the variable into the SpawnActorOfClass node. This will ensure that the clients will spawn the Actor with the variable already set.
Solution 2
Instead of using an RPC (Multicast) set the value in a variable set to “RepNotify”. Then the Clients can print the variable in the OnRep_ function. Replicated variables might skip intermediate values as it only sends the latest value if it can.