Oh... if you are referring to the second level (sewers) that's actually on purpose, i have set max draw distance (geometry culling) to improve performance on mobile. It is configurable.
By coincidence I read up on WebRender on Firefox today and decided to force it turned on - suddenly the game looks nice (only glitch when looking down below the candle one of the wall segments still vanishes).
But it runs a lot better with WebRender (which uses the Graphics card to composite) than without on my machine.