Trying to create a custom solid fuelbox for non-electrical multiblocks and I found that only the first item in the "matches": [] array can be used in the recipe.

Used this following recipe_map json:

{
  "name": "heat_generation",
  "progressTexture": "multiblocked:textures/gui/progress_bar_arrow.png",
  "recipes": [
    {
      "uid": "coal",
      "duration": 100,
      "inputs": {
        "item": [
          {
            "chance": 1.0,
            "content": {
              "amount": 1,
              "matches": [
                "{id:\"minecraft:coal\",Count:1b,Damage:0s}",
                "{id:\"minecraft:coal\",Count:1b,Damage:1s}"
              ]
            }
          }
        ]
      },
      "tickOutputs": {
        "fluid": [
          {
            "chance": 1.0,
            "content": "{FluidName:\"lava\",Amount:2}"
          }
        ]
      }
    }
  ]
}

To confirm my findings, I also switched the coal and charcoal around and when the charcoal was at the top of the list, only charcoal was able to be used to generate heat (or lava fluid in this case, planning to add a hot gaseous 'heat' fluid to implement some heat system.)

Only way I may be able to get around this, is to process both coals into the same item (pulverized coal), or add them both to the same oredictionary.

Versions used:
MC: 1.12.2
Forge: 14.23.5.2860
MixinBooter: 7.0
Multiblock'd: 0.7.7