While trying to create a plugin to prevent map duplication, I came across several weird instances of Recipe.getResult().getType() returning material types that do not reflect what the displayed material types in-game are in a crafting table. This affects unique recipes like the crafting recipes for tool repair, map enlargement, and map & book cloning.

Tool repair & cloning recipes output Material.AIR instead of the actual material.

Map enlargement outputs Material.MAP instead of Material.FILLED_MAP, when minecraft:filled_map is the result displayed in-game.

Below is the simple debug code I was using for testing:

@EventHandler
public void onCraft(PrepareItemCraftEvent e) {
    if (e.getRecipe() != null) {
        Bukkit.broadcastMessage(e.getRecipe().getResult().getType().toString());
    }
}