Upgrade Inertiajs breaks my code

Try showing your app.js file

App.js file

import './bootstrap';
import '../css/app.css';

import { createApp, h } from 'vue';
import { createInertiaApp, Link } from '@inertiajs/vue3'

import { resolvePageComponent } from 'laravel-vite-plugin/inertia-helpers';
import { ZiggyVue } from '../../vendor/tightenco/ziggy/dist/vue.m';
import DefaultLayoutFile from './Layouts/Default.vue'

const appName = window.document.getElementsByTagName('title')[0]?.innerText || 'Laravel';

createInertiaApp({
    title: (title) => `${title} - ${appName}`,

    resolve: (name) => {
        const page = resolvePageComponent(
            `./Pages/${name}.vue`,
            import.meta.glob("./Pages/**/*.vue")
        );
        page.then((module) => {
            module.default.layout = module.default.layout || DefaultLayoutFile;
        });
        return page;
    },

    setup({ el, app, props, plugin }) {
        return createApp({ render: () => h(app, props) })
            .use(plugin)
            .component('Link', Link)
            .use(ZiggyVue, Ziggy)
            .mount(el);
    },
});

You gave lowercase app in setup. The guide says to change to App

https://inertiajs.com/upgrade-guide#setup-arguments