So leaves (e. g. minecraft:spruce_leaves) have two properties: decayable and check_decay.

So I tried to have a different model for decayable=false,check_decay=false:

{
    "variants": {
        "decayable=true,check_decay=false": { "model": "bookshelf" },
        "decayable=true,check_decay=true": { "model": "bookshelf" },
        "decayable=false,check_decay=true": { "model": "bookshelf" },
        "check_decay=false,decayable=false": { "model": "granite" }
    }
}

The resource pack is loading fine but no matter what block state I set with setblock, the model used is always the standard model.

By adding "normal": { "model": "andesite" } I figured out that Minecraft is only using the normal block model. That's very inconsistent as you can set custom blockstate models for all other blocks.