Broken middleware behaviour for reused endpoints

require 'grape'

class Endpoint < Grape::API
  format :txt

  get(:e) { 'Endpoint was called' }
end

class Middleware
  def initialize(app)
    @app = app
  end

  def call(env)
    @app.call(env).tap do |r|
      r.write(' and middleware too')
    end
  end
end

class Protected < Grape::API
  use Middleware

  mount Endpoint
end

class Routes
  def initialize
    @app = Rack::Builder.app do
      map('/n') { run Class.new(Grape::API) { mount Endpoint } }
      map('/o') { run Endpoint }
      map('/p') { run Protected }
    end.freeze
  end

  def call(env)
    @app.call(env)
  end
end

run Routes.new

$  curl localhost:9292/p/e  
Endpoint was called  

$  curl localhost:9292/o/e    
Endpoint was called     

$  curl localhost:9292/n/e    
Endpoint was called

Expected behaviour:
Middleware would run in endpoint mounted to /p/e.
Actual behaviour:
Middleware don't get executed at all.

If I comment out /n/e mapping:

require 'grape'

class Endpoint < Grape::API
  format :txt

  get(:e) { 'Endpoint was called' }
end

class Middleware
  def initialize(app)
    @app = app
  end

  def call(env)
    @app.call(env).tap do |r|
      r.write(' and middleware too')
    end
  end
end

class Protected < Grape::API
  use Middleware

  mount Endpoint
end

class Routes
  def initialize
    @app = Rack::Builder.app do
#     map('/n') { run Class.new(Grape::API) { mount Endpoint } }
      map('/o') { run Endpoint }
      map('/p') { run Protected }
    end.freeze
  end

  def call(env)
    @app.call(env)
  end
end

run Routes.new

$  curl localhost:9292/o/e
Endpoint was called and middleware too
$  curl localhost:9292/p/e
Endpoint was called and middleware too

Expected behaviour:
Middleware would run in endpoint mounted to /p/e.
Actual behaviour:
Middleware is affecting mapping where it should not get executed.

Start by running the provided Ruby reproduction with the three Rack::Builder mappings and compare middleware behavior for /n/e, /o/e, and /p/e. Trace how mounted Grape endpoints and middleware are assembled, then verify that middleware runs for /p/e without affecting the other mappings.