A possible work around until issue is fixed.
Hi all, I was able to reset chapter 18 progress and can replay the mission from start. Hopefully it will not get stuck again.
To reset chapter 18, I started chapter 17 then I quit it. When I pressed Continue chapter 18 it went back to beginning of chapter 18 (Yong reactor part).
EDIT: Just replayed and had no issues this time.
Only "weird" thing that happened in my first run that stalled, was that during the Scruffy instance I was downed, so I left instance while downed instead of respawning then leaving instance.