Unfortunately I haven't captured a video of this yet, but every single time any of my fireteams have attempted to kill Riven, she becomes invincible when she gets down to about 1 zit of health remaining. Once she is invincible, zits can still be popped, but will not do any damage to her, she will auto-stun and change sides the moment she enters a room, and she will auto-pop her own eyes as soon as she opens her faceplate. Her model also changes to the final stand version of her face, with all of her eyes popped. It almost seems like she is entering her final stand state before her health reaches that point, and her behavior bugs out in response. This has happened over 10 times to me at this point, with fireteams consisting of various different players, over several different weeks. Last Wish is completely unbeatable for us at this point.