If the alert isn’t shown, set the show flag later. I guess the alert view hides behind some view when the view creates complexity.

ContentView.swift

import SwiftUI

struct ContentView: View {
    @State var isPresented = false
    var body: some View {
        VStack {
            Image(systemName: "globe")
                .imageScale(.large)
                .foregroundStyle(.tint)
            Text("Hello, world!")
        }
        .padding()
        .onAppear() {
            DispatchQueue.main.asyncAfter(deadline: DispatchTime.now() + 1) {
                isPresented.toggle()
            }
        }
        .alert("Alert", isPresented: $isPresented) {
            Button("OK") {
                isPresented = false
            }
        } message: {
            Text("Alert message.")
        }
    }
}

#Preview {
    ContentView()
}

The point is:

            DispatchQueue.main.asyncAfter(deadline: DispatchTime.now() + 1) {
                isPresented.toggle()
            }