Any evidence the prosecutor uses that is potentially exculpatory must be disclosed to the defendant. So your questions raise why they wouldn't: they think the defense can't find out. And it's arguably likely that they won't. But if they do, it will not only invalidate that single case, it could potentially trigger retrials of hundreds of cases because now a bunch of people know that Officer Flock is on the Brady list.

And one final thing about this... the prosecutor (who has probably said some variation of "ignorance of the law is not an excuse" to try to get a conviction) can't claim ignorance as a defense to a Brady violation. Failing to disclose, whether they knew or not, is a Brady violation.