In 21w03a, if your brightness setting is set to maximum, the world is rendered as if every single block is recieving a full light level 15, despite both the client and server knowing that the actual light level is different. This effect is almost like Night Vision, without the bright fog.

Steps to reproduce:
-Set brightness option to maximum
-The world is now fullbright

The issue only happened once, and cannot so far be reproduced, so it can be closed.