Rails bug when creating using association and scopes?

appointment.rb

class Appointment < ApplicationRecord
  belongs_to :inquiry
  enum appointment_type: { default: 0, q1: 1, q2: 2 }
  scope :q1, -> { where(appointment_type: q1) }
  scope :on_date, -> (date) { where(scheduled_date: date) }
  
  before_validation :set_start_and_end_time
  
  def duration
    [15, 30, 45].sample
  end
  
  private
  
  def set_start_and_end_time
    self.start_time = if last_appointment_on_same_date = Appointment.on_date(scheduled_date).last
      last_appointment_on_same_date.end_time
    else
      scheduled_date.in_time_zone.change(hour: 8) # 08:00 on scheduled_date  
    end
    self.end_time = self.start_time + duration
  end
end

inquiry.rb

# app/models/inquiry.rb
class Inquiry < ApplicationRecord
  has_many :appointments
end

is_this_a_bug.rb

inquiry_params: { description: "Test inquiry" }
appointment_params: { scheduled_date: Time.zone.today }

@inquiry = Inquiry.new(inquiry_params)
@inquiry.transaction do
  @inquiry.save!
  
  # This does not work cause check for last appointment in `set_start_and_end_time` before validation hook will ne scoped with
  # inquiry id and q1 appointment type.
  # `Appointment.on_date(scheduled_date).last` will produce following SQL:
  #   SELECT * FROM appointments WHERE scheduled_date = 2017-10-08 AND inquiry_id = 12345 AND appointment_type = q1 ORDER BY id DESC LIMIT 1
  # @inquiry.appointments.q1.create!(appointment_params)
  
  # This will work ok.
  # `Appointment.on_date(scheduled_date).last` will produce following SQL:
  #   SELECT * FROM appointments WHERE scheduled_date = 2017-10-08 ORDER BY id DESC LIMIT 1
  appointment = @inquiry.appointment.q1.new(appointment_params)
  appointment.save!
end