I am trying to make fake blocks using carved pumpkins, and some blocks have different textures per side.
I made a cube model that I made in blockbench, with a textures object with each side being named north, south, up, etc.
I have default textures for all sides, with each texture being named cube_.
When setting up a python script to quickly go through all blocks, the blocks would only use the default textures.
I wasn't using the texture property, but was using the texture.sub properties, with each one being the previously mentioned model side names. I then added in a texture attribute, which fixed the texture not loading, but none of the sub textures would override.
I tried things like texture.east= and texture.cube_east=, but neither work.

Here is what the gravel file I was testing on looks like:
type=item
items=minecraft:carved_pumpkin
texture=minecraft:textures/block/gravel.png
texture.east=minecraft:textures/block/diamond_ore.png
texture.up=minecraft:textures/block/gravel.png
texture.down=minecraft:textures/block/gravel.png
texture.north=minecraft:textures/block/gravel.png
texture.south=minecraft:textures/block/gravel.png
texture.west=minecraft:textures/block/gravel.png
model=cube.json
nbt.display.Name=Fake minecraft:gravel

The model is just a 16x16x16 cube in a bone called root.