Hello! I must say that your plugin works really wonderful, the only one issue that I have is that I can't add anything placeable (as blocks) into the cauldron, including mushrooms, leaves and other kind of stuff.
Can you in some way override it?
I've tried everything, from simply clicking it to re-defining the leaves/mushrooms as custom items and it didn't work.

I have something like this (I can't also add the bone meal, but I think it is the issue with Bukkit itself, since it doesn't have separate ID for bone meal, sadly :( )

  bone_meal:
    material:
        - INK_SACK
    name: 'Bone Meal'
    
  red_mushroom:
    material:
        - RED_MUSHROOM
    name: 'Mushroom'
    
  brown_mushroom:
    material:
        - BROWN_MUSHROOM
    name: 'Mushroom'
    
  leaves:
    material:
        - LEAVES

You need to have a block next to the Cauldron that would allow your mushrooms to be placed (like dirt).
Then click the side of the Cauldron where the dirt is and it should go into the Cauldron. This is a 1.12 limitation sadly.

The Bone Meal in 1.12 uses a sub id that you can use in the config using INK_SACK,3 but with the sub id for bone meal instead of 3

Okay, thank you!

I think the same goes for various dyes? Is there a numerical ID list for various dyes and other things (because they're all subtypes)?

Yes exactly. Maybe you can still find an old wiki somewhere or a item id list that has all of the ids listed

Thank you very much for your help!
Will try to find it later then...