The Dig map generates inaccessible area

I still have more than 50 monsters remaining, and did not se a betrayal encounter. It did spawn Alva.

Full map:

However when I go to the corner of the boss room:

Suddenly I can see an abyss!

Almost looks like there's a path from the entrance to the boss-room (from the main map) but the entrance covers it?

Bug report reference: 652,030,633