After completing a run on the first episode of the new DLC, I returned to find that the test chamber prep room had bugged out, with many of the walls and floors having disappeared. Upon embarking on another run, I discovered that the same thing had happened to rest of the DLC levels, i.e. inside/during the run.
I assumed that all this would get fixed when I rebooted the game, but nope. The bug persists. And that's pretty much where this bug stopped being amusing. Now I don't want to go any further for fear that my save is somehow permanently corrupted (I have a backup on a flash drive).