Oh! Sorry I misunderstood.
Well I think character will work just as well as Playercontroller because the ownership chain leads to the same connection.
I dont know the answer but i have an idea. I think maybe the way it works is this:
there is a difference between Authority and IsServer nodes. The former asks whether the owner of the connection is an ahthirity or remote role, but a remote client will still be a remote client player regardless of which machine is running the code.
IsServer asks which version of the world the code is running on, rather than the role of the actor’s owning connection (server’s world or client’s world)
So for this function if my theory is right, you could replace the Authority switch with an IsServer branch
OR
You could connect your SRV_GameSave directly to the MainGameInstance cast and maybe that will work.
I may be mistaken about how authority and remote works but its worth a try