Updating create-react-app proxy settings from 1.x to 2.x - reactjs

Question

I've picked up a legacy project, and have been asked to update the create-react-app package from 1.0.3 to the latest.
The problem is, the new proxy settings aren't working; requests that were previously going to my/resource.do are just returning index.html
As suggested in the docs (https://facebook.github.io/create-react-app/docs/proxying-api-requests-in-development#configuring-the-proxy-manually), I took the 'proxy' object in package.json and converted it to instructions in a file called setupProxy.js. The new js file is definitely being read; I just can't get it to work.
Can anyone see where the mistake is here? This is exactly as specified in the docs, but for some reason requests that worked before are now failing.
Any help greatly appreciated
Before (in package.json)
"proxy": {
"/api": {
"target": "http://localhost:9000"
},
"/some/thing": {
"target": "https://example.org:8452/",
"secure": false,
"changeOrigin": true
},
"/some": {
"target": "https://example.org:7324",
"secure": false,
"changeOrigin": true
},
"/.*": {
"target": "https://example.org:8452/",
"secure": false,
"changeOrigin": true
}
}
After (in setupProxy.js):
const proxy = require('http-proxy-middleware')
module.exports = function(app) {
app.use('/api', proxy({ target: 'http://localhost:9000/' }))
app.use('/some/thing', proxy(
{
target: 'https://example.org:8452/',
secure: false,
changeOrigin: true,
logLevel: 'debug'
}
))
app.use('/some', proxy(
{
target: 'https://example.org:7324/',
secure: false,
changeOrigin: true,
logLevel: 'debug'
}
))
app.use(proxy('/.*',
{
target: 'example.org:8452/',
secure: false,
changeOrigin: true,
logLevel: 'debug'
}
))
}