By default, the Mac telnet client tries to autologin (with the mac account username) to the switch without solicitation. With the changes made for stack node autologin, the telnet daemon now processes autologin requests.
This gets you directly to the password prompt, with possibly the wrong login name automatically entered.
This is expected behavior when the client initiate autologin by default.
Use the '-K' option with telnet, which tells it not to attempt an autologin. You can also make this an alias by adding the following line to your bashrc:
alias telnet='telnet -K'