repro.rb

require "bundler/setup"
require 'active_record'
require 'sqlite3'

ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: ":memory:")
ActiveRecord::Schema.define do
  create_table :gift_cards, force: true do |t|
    t.references :checkout, null: false
    t.string :code
  end
  create_table :checkouts, force: true do |t|
  end
end

class GiftCardValidator < ActiveModel::Validator
  def validate(model)
    model.checkout.gift_cards.any?
    puts "gift_cards: #{model.checkout.gift_cards.size}"
  end
end

class Checkout < ActiveRecord::Base
  has_many :gift_cards, dependent: :delete_all, inverse_of: :checkout, autosave: true
end
class GiftCard < ActiveRecord::Base
  belongs_to :checkout
  validates_with GiftCardValidator
end

checkout = Checkout.create!
checkout.gift_cards.create!(code: "sample")
checkout.gift_cards.create!(code: "sample")

# output:
# gift_cards: 0
# gift_cards: 0