Oh it's just the shadow which disappeared. I am sorry, I misunderstood your description on IRC then.
This is not a bug, it's on purpose. The game will automatically disabled shadows when your RenderFPS decreases below a certain value (I forgot which one exactly, but probably something like 20 or 15). The idea is to keep the game playable for you (and others) in terms of RenderFPS while sacrificing the shadows while RenderFPS remains low (they will return later if your RenderFPS have increased in the meantime).