if you do a normal rpc, you should not have any information lost, althought the information transfer might not be instant (it will re-send until the packet arrives)
if you used rpc_unrelieable, i suggest to go back to standard rpc.
i suppose the issue is not in the lost packet, but rather on the scene tree not being the same on both client and server.
I suggest you to run the client from the debug, and use the "Remote" button to see what the scene tree looks like, to check if it is as you expect.
Remember that node names are important: if you create multiple instance of the same scene, Godot will automatically assign a name depending on how many instance there are.
If for any reason (namely lag), the number of instances on the client is different from the server (even for a brief moment), and a new instance is create in that moment, this last instance will have 2 different names on server and client, and will screw everything up.
If this is what's happening, the solution is to give custom name to the instanced node.