Invisible Bear. The second outdoor section of the mission with the underground subway entrances. If you stealth your way perfectly in the first section then the rest of the streets glitch out and don't let you move on. Your Squad stays behind the road blocks at the beginning and you are unable to secure the checkpoint. Total crap that I get punished for doing it perfect by having to reload the last checkpoint to try and get discovered so I can move on. So basically I stealth kill all enemies but 1, let that last guy see me and engage before I break his neck and move on to the next group.