Showing sheet dismiss navigation view child

I have the same problem.

When I commented out the top “TabView”, the problem disappeared.

I've found a solution for that but I truly believe that it's a SwiftUI bug. You must move the navigation link to the upper parent and view.

import SwiftUI

struct ContentView: View {

    @State
    var shownextPage = false

    var body: some View {

        NavigationView{
            ZStack{
                TabView{
                    Tab1(shownextPage: $shownextPage)
                        .tabItem({Text("tab1")})
                    Tab2()
                        .tabItem({Text("tab2")})
                }

                NavigationLink(destination: SecondView(), isActive: $shownextPage) {
                    EmptyView()
                }
            }
            .navigationViewStyle(.stack)
        }
    }
}

struct Tab1: View {

    @Binding
    var shownextPage:Bool

    var body: some View {

        ZStack{
            Button ("Next Page"){
                shownextPage = true
            }
        }
    }
}

struct Tab2: View {

    var body: some View {
        Text("Tab 2 detail")
    }
}

struct SecondView: View {

    @State
    var showSheet:Bool = false

    var body: some View {

        VStack{

            Button {
                showSheet = true
            } label: {
                Text("open Sheet")
                    .padding()
            }
        }
        .sheet(isPresented: $showSheet, onDismiss: nil) {
            Text("hello in sheet")
        }
    }
}

struct ContentView_Previews: PreviewProvider {

    static var previews: some View {
        ContentView()
    }
}

There is a mistake. You need to move your NavigationView into Tab1. Also remove the ZStack.

struct Tab1: View {

    @State var shownextPage = false

    var body: some View {
        NavigationView{
            NavigationLink( destination: SecondView(), isActive: $shownextPage) {
                Text("Next Page")
            }
        }
    }
}

try this "fullScreenCover" instead of "sheet"