Every time you load into a world; the edges of the screen will darken before fading away.

Steps to reproduce:

1. Launch Minecraft Preview (1.21.60.23)
2. Create a new world
3. Observe the edges of the screen

Expected Result:

The Edges of the screen are not dark unless the player is spawned in at a y level lower than -10.

Observed result:

The Edges of the screen are dark no matter what y level the player spawns at, but they do end up fading away with time.

Notes:

This bug relates to MCPE-121983 and was likely an unintended side effect caused by its fix.

If you're having trouble noticing it, here are some example seeds with generally bright surroundings: