diff --git a/lib/action_controller/serialization.rb b/lib/action_controller/serialization.rb
index 3097cdc4..e8d5a420 100644
--- a/lib/action_controller/serialization.rb
+++ b/lib/action_controller/serialization.rb
@@ -30,8 +30,8 @@ module ActionController
         options[:adapter] = false
       end
       serializable_resource = ActiveModelSerializers::SerializableResource.new(resource, options)
-      serializable_resource.serialization_scope ||= serialization_scope
-      serializable_resource.serialization_scope_name = _serialization_scope
+      serializable_resource.serialization_scope ||= options.fetch(:scope) { serialization_scope }
+      serializable_resource.serialization_scope_name = options.fetch(:scope_name) { _serialization_scope }
       # For compatibility with the JSON renderer: `json.to_json(options) if json.is_a?(String)`.
       # Otherwise, since `serializable_resource` is not a string, the renderer would call
       # `to_json` on a String and given odd results, such as `"".to_json #=> '""'`
diff --git a/lib/active_model/serializer.rb b/lib/active_model/serializer.rb
index e8607845..c3193120 100644
--- a/lib/active_model/serializer.rb
+++ b/lib/active_model/serializer.rb
@@ -96,16 +96,18 @@ module ActiveModel
       end
     end
 
-    def self._serializer_instance_method_defined?(name)
-      _serializer_instance_methods.include?(name)
+    # rubocop:disable Style/ClassVars
+    def self.method_added(method_name)
+      @@_serializer_instance_methods ||= Hash.new { |h, k| h[k] = Set.new }
+      @@_serializer_instance_methods[self] << method_name
     end
 
-    # TODO: Fix load-order failures when different serializer instances define different
-    # scope methods
-    def self._serializer_instance_methods
-      @_serializer_instance_methods ||= (public_instance_methods - Object.public_instance_methods).to_set
+    def self._serializer_instance_method_defined?(name)
+      @_serializer_instance_methods ||= (ActiveModel::Serializer.public_instance_methods - Object.public_instance_methods).to_set
+      @_serializer_instance_methods.include?(name) ||
+        @@_serializer_instance_methods[self].include?(name)
     end
-    private_class_method :_serializer_instance_methods
+    # rubocop:enable Style/ClassVars
 
     attr_accessor :object, :root, :scope
 
diff --git a/test/action_controller/serialization_scope_name_test.rb b/test/action_controller/serialization_scope_name_test.rb
index 5ad7251b..ca1ff22f 100644
--- a/test/action_controller/serialization_scope_name_test.rb
+++ b/test/action_controller/serialization_scope_name_test.rb
@@ -158,8 +158,6 @@ module SerializationScopeTesting
       assert_equal expected_json, @response.body
     end
   end
-  # FIXME: Has bugs. See comments below and
-  #   https://github.com/rails-api/active_model_serializers/issues/1509
   class NilSerializationScopeTest < ActionController::TestCase
     class PostViewContextTestController < ActionController::Base
       serialization_scope nil
@@ -171,12 +169,15 @@ module SerializationScopeTesting
         render json: new_post, serializer: PostSerializer, adapter: :json
       end
 
-      # TODO: run test when
-      # global state in Serializer._serializer_instance_methods is fixed
-      # def render_post_with_passed_in_scope
-      #   self.current_user = User.new(id: 3, name: 'Pete', admin: false)
-      #   render json: new_post, serializer: PostSerializer, adapter: :json, scope: current_user, scope_name: :current_user
-      # end
+      def render_post_with_passed_in_scope
+        self.current_user = User.new(id: 3, name: 'Pete', admin: false)
+        render json: new_post, serializer: PostSerializer, adapter: :json, scope: current_user, scope_name: :current_user
+      end
+
+      def render_post_with_passed_in_scope_without_scope_name
+        self.current_user = User.new(id: 3, name: 'Pete', admin: false)
+        render json: new_post, serializer: PostSerializer, adapter: :json, scope: current_user
+      end
 
       private
 
@@ -194,8 +195,6 @@ module SerializationScopeTesting
       assert_nil @controller.serialization_scope
     end
 
-    # TODO: change to NoMethodError and match 'admin?' when the
-    # global state in Serializer._serializer_instance_methods is fixed
     def test_nil_scope
       if Rails.version.start_with?('4.0')
         exception_class = NoMethodError
@@ -210,21 +209,34 @@ module SerializationScopeTesting
       assert_match exception_matcher, exception.message
     end
 
-    # TODO: run test when
-    # global state in Serializer._serializer_instance_methods is fixed
-    # def test_nil_scope_passed_in_current_user
-    #   get :render_post_with_passed_in_scope
-    #   expected_json = {
-    #     post: {
-    #       id: 4,
-    #       title: 'Title',
-    #       body: "The 'scope' is the 'current_user': true",
-    #       comments: [
-    #         { id: 2, body: 'Scoped' }
-    #       ]
-    #     }
-    #   }.to_json
-    #   assert_equal expected_json, @response.body
-    # end
+    def test_serialization_scope_is_and_nil_scope_passed_in_current_user
+      get :render_post_with_passed_in_scope
+      expected_json = {
+        post: {
+          id: 4,
+          title: 'Title',
+          body: "The 'scope' is the 'current_user': true",
+          comments: [
+            { id: 2, body: 'Scoped' }
+          ]
+        }
+      }.to_json
+      assert_equal expected_json, @response.body
+    end
+
+    def test_serialization_scope_is_nil_and_scope_passed_in_current_user_without_scope_name
+      get :render_post_with_passed_in_scope_without_scope_name
+      expected_json = {
+        post: {
+          id: 4,
+          title: 'Title',
+          body: "The 'scope' is the 'current_user': true",
+          comments: [
+            { id: 2, body: 'Scoped' }
+          ]
+        }
+      }.to_json
+      assert_equal expected_json, @response.body
+    end
   end
 end