Maybe not so weird, especially if your former employee ("FE") doesn't have UA status equal to yours.

My guess is that FE knows or guessed that you might be a 1K. And FE probably believes that using your number would earn some benefits (E+ seating? upgrade? etc.) that would otherwise not be available.

And it probably wasn't too difficult for FE to find your MP number while working for you, especially if your UA mail comes to the office or if you keep it written down in an appointment book or other accessible location.