"sandstone" : {
      "brightness_gamma" : 0.70,
      "isotropic" : {
         "down" : true,
         "up" : true
      }, 

When changing properties in blocks.json for texture packs, the brightness_gamma setting doesn't actually change at all. Every other property can be changed except this one. There is no visual change in-game when this setting is updated and applied.

This has been a noted issue from 2+ years ago here

How to replicate:

• Change textures on a block that uses gamma_brightness by default (example sandstone) to be the same as a block that does not use gamma_brightness by default (example stone)
• Change gamma_brightness setting in blocks.json on sandstone
• See there is no change

Files attached:

Sample file that changes the textures of stone/sandstone to share a texture, and also the modified blocks.json file that should change gamma_brightness if it was working correctly.