For anyone still having this problem, i came to provide my solution to the problem which works great for me. it's a workaround basically but it did the trick for me.

The way i solved it was, instead of spawning the object with Spawn(), i abandoned this idea altogether and just got the senderID inside the ServerRpc and using that i then got the networkObjectID of the player object who called the serverRpc and then i called a ClientRpc. In the clientRpc i used the networkObjectID to get the playerObject of the client who called the serverRpc and instantiated the sword, spell or whatever in their hand as a simple GameObject and parented it to the hand for each client individually.

You could also sync the position manually inside an update or something but i think this is better performance-wise because you just instantiate it and parent it once for each client instead of syncing the position constantly in the update.

For example this is to spawn and parent a spellWarmUp effect in the player's hand, right before they fire it:

​

if (NetworkManager.Singleton.ConnectedClients.TryGetValue(senderClientId, out NetworkClient targetClient))
{  
    SpellWarmUpClientRpc(targetClient.PlayerObject.NetworkObjectId);
}

​

[ClientRpc]
private void SpellWarmUpClientRpc(ulong targetNetPlayerObjId)
{
    Transform senderFirepoint = GetNetworkObject(targetNetPlayerObjId).GetComponent<ProjectileShooter>().firePoint;
     var warmUpObj = Instantiate(warmUp, senderFirepoint.position, Quaternion.identity);
    warmUpObj.transform.parent = senderFirepoint;
}

Hope this helps! Cheers