I got no headcrabs, either. I saw them spawn at the top of the slope, but they simply walked back into the room above and never attempted to jump down at me.
Furthermore, there’s an extra bug with this: The music that plays is tied to pressing the button, but you can press the button multiple times on the way down and the music restarts each time. I believe the button should become inert as soon as it’s pressed the first time so that it can’t be touched while the elevator is moving. Either that, the music should be prevented from restarting itself. It’s supposed to be background mood and it comes off like it’s playing in the PA system.
That’s actually the only obvious bug I’ve seen.