Node后端+Vue(Vite)前端Axios请求CORS错误求助

我了解该问题在Stackoverflow上已有大量讨论，但仍未找到解决办法。简而言之，我基于Node.js开发了对接MailChimp的API，并在Vite构建的Vue.js客户端应用中调用该API。此API在Postman中可正常工作，但前端调用时返回错误：

Access to XMLHttpRequest at from origin has been blocked by CORS policy: Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource

服务端index.js代码 #

const cors = require('cors')
app.use(cors({ origin: '*' }))
app.use('/v1/crm/leads', require('./src/routes/LeadRoutes'))

控制器方法代码 #

await addSubscriberToAudience('xxx', req.body.email).then(async (mailchimpID) => {
  if (mailchimpID) {
    let data = {
      references: {
        mailchimpID: mailchimpID
      }
    }
    let updatedLead = await Lead.findByIdAndUpdate(lead.id, data, { new: true })
    res.status(200).json({ data: updatedLead, message: 'New Lead Created Successfully' })
  } else {
    console.log('Null Mailchimp ID')
  }
})

前端调用代码 #

axios.post(APIURL + '/v1/crm/leads/create', data, {
        headers: { 'X-ORIGIN': 'https://example.com' }
      }).then((response) => {
        console.log(response)
      }).catch((error) => {
        console.log(error.message)
      })

我能看到预检请求的响应头已设置Access Allow Origin：

后端部署在AWS Elastic Beanstalk，前端部署在AWS S3 Bucket，恳请帮助，谢谢！

补充说明1：
网站对后端的其他POST API调用均正常，仅该接口异常。差异在于此接口是客户端调用后端后，后端先调用MailChimp API，等待响应后再返回前端，望此背景有助排查。

内容的提问来源于stack exchange，提问作者Jacky.S