I've heard aboout this bug, the background process that is used to load the new layer's blocks can be interupted by the game not being the active window, resulting in no blocks being loaded, did you switch windows or alt tab by any chance?
Sorry that that happened, for now it can only be fixed by reloading the page or resetting the save file, I'm currently looking for a solution on this :)
I also experienced the same issue when trying to fight an enemy, he just dug underground to avoid fighting