May have been browser cache issue of some sort. I was able to reproduce it, but only once. Then I restarted the browser to take some screenshots. Now the level to the left behaves differently. Sounds like "out of bounds", as you said. I can still sort of move the character to that location - it goes off the screen and I'm stalled until I hit page-reload. But a) it doesn't load the map in the screenshot, and b) page-reload resets the entire map, including character position, to what they were when I first entered it.
Kinda weird that I was able to reproduce it once - PC was off overnight. But I'm clueless re cache behavior. Maybe I refreshed the page whilst taking screenshots - at the time I wasn't suspecting a cache issue and wasn't paying much attention to that.