Yeah, last night my friends and I discovered this on our 140 day world, when we got the basement legit. the stairs were broken too, sometimes inside of the wall making it impossible to escape, We retried it many times in different locations, and still the same issue

It must have been one of the new updates, cause I swear I was messing around with basements a few months ago.

It almost seems like only a quarter of the basement is generating properly.
Perhaps the world size has changed, or the functions reporting it have changed, because I just made a test world set to the max size and the basement worked fine. Even mining the walls was functioning no problem.

On the default world size, however, it was broken exactly like this; only 2 corner sides of the basement generate, with alternating normal and missing walls, and no stairs.

Some more information:
I just tested mining to the extents of the basement, and after a while and going along the edge, parts of the walls started to disappear. Parts of the floor did, as well.
I don't entirely know what this means, maybe some syncing issue?
If the author is willing to come back and fix this, maybe this bit of information can help.

Going out and then back in to the basement seemed to restore the floor and walls, along the entire path, but mining more made some parts disappear again.
Which parts were missing seemed to 'shuffle' with every new section mined.
Perhaps the way things sync has changed, or there's a new maximum? If the basement's walls and floors get regenerated each time it changes, which is what it looks like is happening, perhaps it just struggles to sync?
In the backspace debug menu, I noticed my entity count rising dramatically with each wall mined. Maybe there's just too much going on nowadays?