What version are you on? Also are you using an old save?
The version I posted the comment on is what I was using. And nope did a brand new save/clean slate.
Played some today with the updated version and had a dark passageway with the same layout and I could use the stairs still. So maybe fixed with new version?
Edit:saw my comment on main page instead.
I believe it was 5.4.30 (maybe patch 1)