Okay, so I think I found a solution. After defeating the Eater of Souls, don't be hasty trying to get to the gate as quickly as possible. Have the final dialogue with Joko, standing a little bit to the right of Joko's cell, away from the portal. Once you receive the quest update to go through the portal after your character says "But I won't need you," then you can book it to open the gate and jump through the portal as fast as possible. I was booted out of this mission 5 times at this exact point, but only got it once I did this.
Okay, so I think I found a solution. After defeating the Eater of Souls, don't be hasty trying to get to the gate as quickly as possible. Have the final dialogue with Joko, standing a little bit to the right of Joko's cell, away from the portal. Once you receive the quest update to go through the portal after your character says "But I won't need you," then you can book it to open the gate and jump through the portal as fast as possible. I was booted out of this mission 5 times at this exact point, but only got it once I did this.