IdentityPoolRoleAttachment资源无法更新

我使用以下CloudFormation来创建堆栈

Resources:
    MyIdentityPool:
      Type: AWS::Cognito::IdentityPool
      Properties:
        AllowUnauthenticatedIdentities: 'true'

    CognitoRole:
      Type: 'AWS::IAM::Role'
      Properties:
        AssumeRolePolicyDocument:
          Version: '2012-10-17'
          Statement:
            - Effect: Allow
              Principal:
                Federated:
                  - 'cognito-identity.amazonaws.com'
              Action: sts:AssumeRoleWithWebIdentity
              Condition:
                StringEquals:
                  cognito-identity.amazonaws.com:aud:
                    Ref: MyIdentityPool
                ForAnyValue:StringLike:
                  cognito-identity.amazonaws.com:amr:
                    - 'authenticated'        

    CognitoRole2:
      Type: 'AWS::IAM::Role'
      Properties:
        AssumeRolePolicyDocument:
          Version: '2012-10-17'
          Statement:
            - Effect: Allow
              Principal:
                Federated:
                  - 'cognito-identity.amazonaws.com'
              Action: sts:AssumeRoleWithWebIdentity
              Condition:
                StringEquals:
                  cognito-identity.amazonaws.com:aud:
                    Ref: MyIdentityPool
                ForAnyValue:StringLike:
                  cognito-identity.amazonaws.com:amr:
                    - 'unauthenticated'

    RoleAttachment:
      DependsOn: MyIdentityPool
      Type: 'AWS::Cognito::IdentityPoolRoleAttachment'
      Properties:
        IdentityPoolId:
          Ref: MyPinpointIdentityPool
        Roles:
          unauthenticated:
            Fn::GetAtt: CognitoRole2.Arn


    AccessPolicy:
      Type: 'AWS::IAM::Policy'
      Properties:
        ...
        Roles:
          - Ref: CognitoGeneralRole

堆栈创建成功但是当我通过将IdentityPoolRoleAttachment更改为更新堆栈时

RoleAttachment:
      DependsOn: MyIdentityPool
      Type: 'AWS::Cognito::IdentityPoolRoleAttachment'
      Properties:
        IdentityPoolId:
          Ref: MyPinpointIdentityPool
        Roles:
          unauthenticated:
            Fn::GetAtt: CognitoRole2.Arn
          authenticated:
            Fn::GetAtt: CognitoRole.Arn

我得到 AWS::Cognito::IdentityPoolRoleAttachment Resource cannot be updated .

可能是什么原因？