Why doesn't unscope work for HABTM in the includes request?

Question

Ruby on Rails

Hi everybody.
There are 2 models.

class User < ActiveRecord::Base
  has_and_belongs_to_many :tasks
  has_and_belongs_to_many :completed_tasks, -> { unscope(where: :completed).where(tasks: {completed: true}) }, class_name: Task, join_table: :users_tasks
end

class Task < ActiveRecord::Base
  default_scope -> { where(completed: false).order(id: :asc) }

  has_and_belongs_to_many :users
end

The code works fine, however the code
User.find(1).completed_tasks

User.includes(:completed_tasks).find(1).completed_tasks

returns nothing. When viewing the log, it is clear that the following conditions are present in the SQL query:

LEFT OUTER JOIN "tasks" ON "tasks"."id" = "users_tasks"."task_id" AND "tasks"."completed" = 'f'
WHERE "tasks"."completed" = 't' AND "users_tasks"."user_id" IN (17)

That is, defalut_scope remains for JOIN tables. How to correctly write unscope for table join?
Thanks in advance.