MCPE-167836 was fixed in 1.20.60.22 Preview, which means brightness_gamma values can darken the block correctly.

However it can not affect the glowing blocks.

For example, we apply a resource pack, which includes the blocks.json and we edit the sand and glowstone in it, set a same brightness_gamma value.

Like this, set the value for 20.

{
  "format_version": [
    1,
    1,
    0
  ],
  "glowstone": {
    "brightness_gamma": 20
  }, 
  "sand": {
    "brightness_gamma": 20
   }
}

Activate the pack and load a world, place a sand and a glowstone.

Then we can see the sand side is dark while the glowstone is still light like before, see the attachment screenshot.

In vanilla bedrock resource pack, crying obsidian have a brightness_gamma value, which is 2.00 (default is 1). Because of the bug, it can not affect this block in game.