When teleporting to a freshly-created z level, the webclient will display a black screen. Once the client has exited the new z level, the screen will begin rendering again. This behavior happens exclusively on the webclient, I should clarify.
Code:
world.maxz = 5
loc = locate(1, 1, 5)
I also went back to BYOND 508 and this wasn't an issue. Once I have more time to manually test each BYOND version one by one backwards to see where the issue cropped up.