Accessing localhost from SvelteKit SSR is suddenly broken after upgrading to Node 18

I have a SvelteKit website that I run on localhost:3000, and a Django API that I run on localhost:8000. The SvelteKit website talks to the API by accessing localhost:8000, both in SSR and in the client, and this has always worked perfectly fine. Until today.

All of a sudden this no longer works and I get this error in console when trying to make a request to the API from SSR:

TypeError: fetch failed
    at fetch (/Users/kevin/Workspace/project_name/node_modules/.pnpm/undici@5.22.1/node_modules/undici/index.js:109:13)
    at process.processTicksAndRejections (node:internal/process/task_queues:95:5)
    at async Promise.all (index 2)
    at async fetchUserMiddleware (/Users/kevin/Workspace/project_name/src/middleware.ts:59:5)
    at async copyCookieValues (/Users/kevin/Workspace/project_name/src/middleware.ts:21:10)
    at async redirectMiddleware (/Users/kevin/Workspace/project_name/src/middleware.ts:16:10)
    at async /Users/kevin/Workspace/project_name/node_modules/.pnpm/@sentry+sveltekit@7.56.0_@sveltejs+kit@1.20.4_svelte@3.59.2/node_modules/@sentry/sveltekit/cjs/server/handle.js:111:19 {
  cause: Error: connect ECONNREFUSED ::1:8000
      at TCPConnectWrap.afterConnect [as oncomplete] (node:net:1494:16)
      at TCPConnectWrap.callbackTrampoline (node:internal/async_hooks:130:17) {
    errno: -61,
    code: 'ECONNREFUSED',
    syscall: 'connect',
    address: '::1',
    port: 8000
  }
}

I have made no code changes, but I did recently run brew upgrade, and I updated pnpm. I'm running Ventura 13.4.1, and I'm not using Docker or anything like that.

When I manually access localhost:8000 in the browser, or via an API tool like RapidAPI, it all works perfectly normal, no problem. Requests from SvelteKit from the browser also work fine, only the ones from SSR are not working.

When I change my SvelteKit website to make requests to 127.0.0.1:8000 instead of localhost:8000, then SSR requests work again. So I have a solution, but I'd like to know what the hell is going on. What suddenly changed and how can I make it work again?

Reverting the code to an older version makes no difference. Like I said, this used to work perfectly fine last time I worked on the project last Friday, and now two days later all of a sudden it gives this error with no changes in the code.

Edit: I also recently updated from Node 16 to Node 18. Downgrading back to 16 fixes it. Why does Node 18 break this and how do I make it work again?

Ok, apparently there was a change in Node 17 that changes the order of what localhost resolves into, see https://github.com/nodejs/undici/issues/1602. Django by default doesn't listen to IPv6. So I could either always start Django on IPv6, or just change localhost to 127.0.0.1, which I've gone with for now.

Edit:

A third option is to include this code in hooks.server.ts:

import dns from "node:dns";
dns.setDefaultResultOrder("ipv4first");