Sveltekit not redirecting from unforeseen route - sveltekit

Question

This is my code for /+layout.ts
export const ssr = false;
export const load = (async ({ url }) => {
let user: null | { user: User, player: SupaPlayer } = await supa.getUser((event, session) => {
...
});
if (user !== null) {
...
return { ...user, contestants: await supa.getContestants() };
} else {
if (!url.pathname.includes('/login')) {
// I AM REACHING THIS POINT
throw redirect(301, "/login");
}
// return null;
}
}) satisfies LayoutLoad
I am trying this with a route that does not exist (i.e. there is nothing in /routes) and get two errors:
in the terminal that I am running my code for dev I get Error: Not found: /<route> and a stacktrace - indeed the route does not exist in my code but I would expect Sveltekit not to create errors for that
in the browser, I get error: Not found: /<route> attributed to manifest.js, and, more importantly, the redirect to /login is not working, and I get Uncaught (in promise) Redirect {status: 301, location: '/login'}
I suspect the initial errors are causing the redirect to fail, as it does work for known routes.
The error seems to originate from manifest.js, which reads as below and which has no obvious default handler for an unknown route
export { matchers } from '/.svelte-kit/generated/client/matchers.js';
export const nodes = [() => import("/.svelte-kit/generated/client/nodes/0.js"),
() => import("/.svelte-kit/generated/client/nodes/1.js"),
...];
export const server_loads = [];
export const dictionary = {
"/": [2],
...
};
export const hooks = {
handleError: (({ error }) => { console.error(error) }),
};