a.rb

require 'active_support'

module M1
  extend ActiveSupport::Concern
  module ClassMethods
    def cm
      puts 'M1.cm'
    end
  end
end

module M2
  extend M1
end

class C
  include M2
end

#C.cm
#xxxx:20:in `<main>': undefined method `cm' for C:Class (NoMethodError)

# WHY?

module M3
  extend ActiveSupport::Concern
  included do
    include M1
  end
end

class C2
  include M3
end

C2.cm
#=> M1.cm