You were right above. I just tested it. Mosh sets the -S flag as none by default on ssh and disables controlmaster. If you use --experimental-remote-ip=remote u can ask the server for an address and the controlmaster config will work.

If the host is behind a NAT then use =local instead of =remote.

im also currently making it so mosh-rs prints a hint when it sees a ControlMaster socket the default mode is about to bypass. should be out soon.