That’s well known issue. It’s not a good idea to use RPCs on BeginPlay due to delay with actors spawning on clients.
For initialization you can use ReplicatedWith (RepNotify) properties.
For example you can create ReplicatedWith (RepNotify) boolean property “IsInitialized”, specify its replication notifier function and set its value to True in BeginPlay on server. Then, when actor will be spawned on the clients, IsInitialized will be replicated and your replication notifier will be called on every client. So this is a good moment to continue initialization on the clients and from this point you can use RPCs for sure.
Best,
Andy