GraphqlPlayground::Rails::Engine Server cannot be reached on Heroku

Question

Locally GraphQL Playground is running fine, but when deployed to Heroku it starts to show error message below as well as 'Server cannot be reached' status.

When I try to call queries and mutations they are working as expected, but error is still visible.

routes.rb

mount GraphqlPlayground::Rails::Engine, at: "/graphql_playground", graphql_path: "graphql#execute"

initializers/graphql_playground.rb

GraphqlPlayground::Rails.configure do |config|
  config.headers = {
    'X-Auth-Header' => ->(view_context) { "123" }
  }
  config.title = "Playground"
  config.csrf = true
  config.playground_version = "latest"
  config.settings = {
    "schema.polling.enable": false
  }
end

Is that a configuration problem or there is something else to it?

Screenshot GraphQL Playground