Working with Form Object and nested attributes: Couldn't find JoinTableRecord with ID=1 for Parent with ID=

Question

I'm working on a form object where I build and save an instance of Service, but I'm having a hard time building and saving nested records.

A Service has_one features_set and accepts_nested_attributes_for for it.

A FeaturesSet accepts_nested_attributes_for :crown_items, which is a join table between that model and Crown.

features_set.rb

class FeaturesSet < ApplicationRecord
  has_many :crown_items
  has_many :crowns, through: :crown_items

  accepts_nested_attributes_for :crown_items
end

crown_item.rb

class CrownItem < ApplicationRecord
  belongs_to :features_set
  belongs_to :crown
end

crown.rb

class Crown < ApplicationRecord
  has_many :crown_items
  has_many :features_sets, :through => :crown_items
end

This is what my ServiceForm form object looks like:

class ServiceForm
  include ActiveModel::Model
  include ActiveModel::Validations::Callbacks

  attr_accessor   :crowns
                  
  before_validation :build_features_set

  def initialize(params = {})
    super(params)
  end

  def save
    return false if invalid?

    service.save
  end

  def service
    @service ||= Service.new
  end

  def build_features_set
    service.build_features_set(
      crown_items_attributes: crowns
    )
  end
end

On initialization, this is what the crowns param looks like:

{0=>{:quantity=>1, :crown_id=>1}, 1=>{:quantity=>5, :crown_id=>2}}

When I try to save the service instance, I get the following error:

Couldn't find CrownItem with ID=1 for FeaturesSet with ID=

and now if I access the crowns params again, it looks like this:

{0=>{:quantity=>1, :crown_id=>1, "id"=>"1"}, 1=>{:quantity=>5, :crown_id=>2, "id"=>"1"}}

It looks like it is creating the crown_item records as it's adding an ID to them (albeit they both have the same ID), but I don't really understand what the error is telling me.