Rails `after_create` <> `has_many through` weirdness

app.rb

# frozen_string_literal: true

require "bundler/inline"

gemfile(true) do
  source "https://rubygems.org"

  gem "rails"
  # If you want to test against edge Rails replace the previous line with this:
  # gem "rails", github: "rails/rails", branch: "main"

  gem "sqlite3"
end

require "active_record/railtie"
require "minitest/autorun"

# This connection will do for database-independent bug reports.
ENV["DATABASE_URL"] = "sqlite3::memory:"

class TestApp < Rails::Application
  config.load_defaults Rails::VERSION::STRING.to_f
  config.eager_load = false
  config.logger = Logger.new($stdout)
  config.secret_key_base = "secret_key_base"

  config.active_record.encryption.primary_key = "primary_key"
  config.active_record.encryption.deterministic_key = "deterministic_key"
  config.active_record.encryption.key_derivation_salt = "key_derivation_salt"
end
Rails.application.initialize!

ActiveRecord::Schema.define do
  create_table :users do |t|
    t.text :name
  end

  create_table :roles do |t|
    t.text :title
  end

  create_table :user_roles do |t|
    t.belongs_to :user
    t.belongs_to :role
    t.index [:user_id, :role_id], unique: true
  end
end

class User < ActiveRecord::Base
  # placing assign_default_role here causes unique constraint errors
  after_create :assign_default_role

  has_many :user_roles
  has_many :roles, through: :user_roles

  # placing assign_default_role here works for unknown-to-me reasons
  # after_create :assign_default_role

  def assign_default_role
    role = Role.first
    roles << role
  end
end

class Role < ActiveRecord::Base
  has_many :user_roles
  has_many :users, through: :user_roles
end

class UserRole < ActiveRecord::Base
  belongs_to :user
  belongs_to :role
end

class BugTest < ActiveSupport::TestCase
  def test_association_stuff
    role = Role.create!(title: "admin")
    user = User.create!(name: "Derpman")

    assert_equal 1, user.roles.count
    assert_equal user.id, role.users.first.id
  end
end

Issue described in this forum post

https://discuss.rubyonrails.org/t/order-of-after-create-callback-and-has-many-through-causes-a-database-constraint-violation/88719