In 1.21.50.28 Preview, the texture of nether bricks has been updated in texture file. However, due to spelling error, the new texture is unused actually.

Code Analysis:
The texture definition of nether bricks from /resource_packs/vanilla_1.16/blocks.json:

  "nether_brick": {
    "textures": "nether_brick",
    "brightness_gamma": 0.80,
    "sound": "nether_brick"
  }

The file path of new texture is /resource_packs/vanilla_1.21.50/textures/blocks/nether_bricks.png. The new texture is not named as nether_brick.png, causing unused texture.

How to Fix:
Changing the file name of new texture from nether_bricks.png to nether_brick.png, or changing the texture definition from nether_brick to nether_bricks will resolve this issue.