以yaml swagger内在功能导出的AWS API网关不起作用

Question

我在cloudformation模板中定义了aws apigateway，如下所示：

  PptApi:
    Type: 'AWS::Serverless::Api'
    Properties:
      StageName: !ImportValue StageName
      DefinitionBody:
          'Fn::Transform':
            Name: 'AWS::Include'
            Parameters:
              Location: !Join [ "", [ "s3://", !Ref S3APIBucket, "/api_ppt.yaml" ] ]

摇摇欲坠的Yaml模板如下：

---
swagger: "2.0"
info:
  version: "2018-06-26T08:03:26Z"
  title:
      Fn::Join: ["", ["full-ppt-", "${Fn::GetAtt: PptApi.StageName}"]]

basePath:
  Fn::Join: ["", ["/", "${Fn::GetAtt: PptApi.StageName}"]]

它会很好地创建API网关，除了标题解析为：full-ppt-${Fn::GetAtt: PptApi.StageName}而不是full-ppt-demo

但是，basePath会按预期解析为/demo（这是阶段名称变量）。为什么它在一种情况下起作用而在另一种情况下不起作用？