Yeah, that's a bit odd...
Obviously you figured it out by now - but the key is just to go back and forth, and keep interacting with the "doors" at each end of the hallway as it shrinks down. After a few times, it just becomes automatic.
Tried it again myself, and no issues. We also had a streamer check out the game last night, with no issues on her end. So it's either a strange one-time bug or a trigger was missed.
Regardless, I appreciate you bringing this to my attention. It's something we'll definitely keep in mind if we do a similar event in the full game. As for the Demo, If this proves to be an issue for others down the line, I'll go in and try to simplify it a bit.