404s/+error.svelte incorrectly handled on Vercel when prerendering

[madeleineostoja]

I feel like I'm missing something extremely basic so I wanted to open this as a discussion before making a repro for a proper issue — I have a basic site deployed to vercel with adapter-vercel, and if I set export const prerender = true; from my root +layout.server.ts, the site no longer renders +error.svelte for 404s, instead showing Vercel's standard 404 page.

Am I missing something? Or does +error.svelte not work when prerendering your whole route tree