As the title says, when connecting to a locally hosted multiplayer server, the saved uplink is not used.
To reproduce:
# start a dedicated server with authentication enabled
# start a client on the same machine, uplink manually[make sure to tick `save credentials`], and connect to localhost:4242
# everything works fine, close the client
# start the client again, and connect to localhost:4242 again, this time without uplinking manually
# observe as your connection is refused as the username is protected, and your credentials are unused.