"Custom_Model_Data" NBT does not work for anything other than items which also includes the block's inventory item but You cannot place down a block with a custom model data NBT. I am thinking it is because blocks lose their NBT when placed but it didn't work with entities either such as string placed down and chests which do use some NBT please fix this in the future or implement it (It might not be a bug)

NOTE: All blocks spammed in image below have the custom_model_data tag and are configured properly in texturepack

Here is an example:

{    
     "parent": "block/cube_all",    
     "textures": {
        "all": "block/iron_block"
    },
     "overrides":[
       { "predicate":{"custom_model_data":987},"model":"block/cookie"} 
    ]
}