Hey there, today I was working on my datapack and I've noticed that stonecutter recipes can't give an item with one or more NBT tags in their result. Here's the JSON I wrote for my recipe.

{
    "type": "minecraft:stonecutting",
    "ingredient": {
        "item": "minecraft:iron_ore"
    },
    "result": "minecraft:turtle_egg",
    "data": "{CustomModelData:1}",
    "count": 2
}

This is fixed in the 1.15-2.0 branch.
Either check out the alpha versions or wait until it gets more stable 😉.