It seems that the wave effect doesn't quite fully finish its animation... or something. When you take a tile warp with wave effects, part of the black border around the game screen is a shade lighter than it should be. Taking a tile warp or something like this will fix it.
EDIT: This even occurs in 2.10.
Not confirmed! The difference in shades is negligible if not nonexistant.
Very negligible. It might make a difference if your monitor is running on 16-bit or 32-bit colors. I can tell the difference between two incredibly close shades of a color, which is why I could detect this. I'll try to see if it's still in the current beta.
Is the bug affecting certain layers? I can't confirm it in my test quest.