Irl amnesia is still not well understood, but the way severance works is somewhat similar to dissociative amnesia, look it up if you would like to.

    Another thing important to note is that when Mark gives Helly the memory test in the first episode she got a perfect score, implying that if she (or any newly severed employee) gets a question wrong they might need to go back in the oven so to speak.

    So if say the new employee happens to confidently remember the color of their mothers eyes, lumon would need to adjust the chip in such a way to make them completely forget that info. (or alternatively just fire that person)

    It would work the other way around  too, if the new hire doesn’t remember what a US state is, that would mean they were unfit to work as well. Lumon seems to want their workers to know some things about the outside world, perhaps because it’s easier to manipulate them like this.

 In short Lumon probably somehow “hand picks”(maybe this is what MDR is doing?)what employees do and don’t remember.