Can't access headers.origin in NextJS middleware (App router)

I'm trying to follow the basic tutorial provided by NextJS to setup a middleware for my API. When trying to restrict origin (for testing purposes) to http://localhost:3000, the request.headers.get('origin') won't return anything, triggering my custom CORS error.

import {NextRequest, NextResponse} from "next/server";

const allowedOrigins = [
    'http://localhost:3000',
]

const corsOptions = {
    'Access-Control-Allow-Methods': 'GET, POST, PUT, DELETE, OPTIONS',
    'Access-Control-Allow-Headers': 'Content-Type, Authorization',
}


export function middleware(request: NextRequest) {

    const origin = request.headers.get('origin') ?? ''
    const isAllowedOrigin = allowedOrigins.includes(origin)

    // Handle simple requests
    const response = NextResponse.next()

    if (isAllowedOrigin) {
        response.headers.set('Access-Control-Allow-Origin', origin)
    } else {
        return Response.json({ message: 'CORS error' }, { status: 533 })
    }

    Object.entries(corsOptions).forEach(([key, value]) => {
        response.headers.set(key, value)
    })

    return response
}

export const config = {
    matcher: '/api/:path*',
}

When trying to log origin before verification, it shows a blank value. My frontend calls are simple fetch functions, nothing fancy here.

By logging the request, I can clearly see origin: 'http://localhost:3000'.

EDIT: after further testing, I noticed that it seems to be the case only with GET requests.