The bug

When within five blocks of a world border (on the inside of the border), brightness greatly increases. The light level does not change, but the brightness of what is displayed is increased. This is most noticeable in the end, but can also be reproduced in the nether and the overworld.

This is likely caused by the overlay applied to the player when close to the border.

How to reproduce

1. Run the command:

   /execute in the_end run tp @s 29999980 100 ~

2. Move toward and away from the border
   The brightness changes when closer than five blocks from the border

Video