Is shapeless recipe in Minecraft really shapeless? See MinecraftForge/MinecraftForge#4472 for details. This has been fixed.

shapeless_recipe_is_not_really_shapeless.json

{
  "group": "testing",
  "type": "minecraft:crafting_shapeless",
  "result": {
    "item": "minecraft:diamond"
  },
  "ingredients": [
    {
      "_comment": "this is needed to avoid potential recipe conflict",
      "item": "minecraft:nether_star"
    },
    [
      {
        "item": "minecraft:planks",
        "data": 0
      },
      {
        "item": "minecraft:planks",
        "data": 1
      },
      {
        "item": "minecraft:planks",
        "data": 2
      },
      {
        "item": "minecraft:planks",
        "data": 3
      },
      {
        "item": "minecraft:planks",
        "data": 4
      },
      {
        "item": "minecraft:planks",
        "data": 5
      }
    ],
    {
      "item": "minecraft:planks",
      "data": 1
    }
  ]
}