Thanks for your feedback!
The shadow (other half) disappearing is normal as the shadow isn't cast on glass (blue transparent tiles), only on black tiles. So it might jump around. But the sudden blinking isn't normal.
How smooth was your experience in terms of framerate? I had the same bug a lot when I didn't have hardware acceleration in my browser and the game was running at 10 fps. After turning on hardware acceleration, the game ran smoothly and the issue was fixed for me so I didn't investigate it further.
I hope you didn't get that bug too often. Otherwise I'm sorry for the frustrating experience and I will look into what's causing this.