Using rhino-forge-1902.2.2-build.268, kubejs-forge-1902.6.1-build.9999 (build.236 with KubeJS-Mods/KubeJS#646 applied), kubejs-create-forge-1902.2.4-build.11, architectury-6.5.85-forge and forge 43.2.14

Using this code

event.recipes.create.milling("cobblestone", "kubejs:layer_1")

the recipe shows up in jei, and the item is accepted into the millstone, but it doesnt ever complete and produce an output. Some particles do show, but not as many as other recipes.
Using this code with event.custom does work, the recipe shows up in jei and

  event.custom({
    "type": "create:milling",
    "ingredients": [
      {
        "item": "kubejs:layer_1"
      }
    ],
    "processingTime": 200,
    "results": [
      {
        "item": "minecraft:cobblestone"
      }
    ]
  })

It seems it caches the last recipe too, so to test it accurately you need to mill another default recipe after each /reload to clear the cache.

Try adding .processingTime(200)

Fixed by implementing a default value that is always written for required recipe types (#35)