This is pretty sophisticated. YOu need to check with the language of the plan to see if your “normal retirement date” is specifically linked to your Social Security Full Retirement Age.
It is really up to the HR department or plan sponsor to answer those questions for you. And to see if they will/can waive the penalty if they really want to keep you on. Sounds like a steep penalty for continuing to work past whatever they see as ‘normal’ retirement age. I can understand perhaps no future contributions — but a forfeiture of benefits at retirement is unusual. Perhaps, they are just warning you that if you continue to earn money as a salary, you won’t receive pension benefits for that month.
You must sort it out with them because they know –and are obliged to provide education –re the details.