The bug

By the world barrier (~4 blocks away) the game's normal black vignette in unloaded chunks disappears, probably to make room for the red one near the barrier. That's about it. Its not a huge problem, but its disorienting.

How to reproduce

1. Create a dark area

   /fill ~ ~5 ~ ~20 ~5 ~20 sandstone
   

2. Set the world border

   /worldborder center ~ ~
   

   /worldborder set 16
   

3. Move towards the center of the dark area
   The corners of your screen suddenly become bright (vignette is removed) before turning red