不推荐以这种方式使用NavigationLink，所以我并不奇怪它会导致错误的行为。这是由于您的RegistrationView没有放在NavigationView（已弃用）或NavigationStack中，因为这些视图提供了导航链接的大部分功能。
就像你说的，NavigationLink的这种用法已经过时了。在我看来，isActive属性一直有点模棱两可（据我所知，它并不是导航链接的“激活器”，而是一种读取链接是否活动的方式）。显示导航链接的新方式（使用.navigationDestination）要好得多。

使用布尔属性显示视图

实际上，你需要的是当一个布尔属性被切换为true时，显示ProfilePhotoSelectorView。这是SwiftUi中的常见范例，有很多方法可以实现这一点，比如.sheet(isPresented:content:)或.popover(isPresented:content:)。注意，这两个方法中的isPresented参数都是布尔属性。例如，使用.sheet：

struct RegistrationView: View {
    // ...
    @EnvironmentObject var viewModel: AuthViewModel
    
    var body: some View {
        VStack {
            // ...
        }
        // Presents the photo selector view when `didAuthenticateUser` is true
        .sheet(isPresented: $viewModel.didAuthenticateUser) {
            ProfilePhotoSelectorView()
        }
    }
}

向导航树添加新视图

如果您坚持使用导航链接（也就是说，您 * 真的 * 希望ProfilePhotoSelectorView成为导航树中的一个节点），那么您必须学习使用新的NavigationStack并将视图追加到路径上。here和here是一个很好的起点）视图模型是最有可能控制堆栈的地方，尽管你最终可能想创建一个专用的视图模型。

struct ContentView: View {
    @StateObject var viewModel = AuthViewModel()
    
    var body: some View {
        NavigationStack(path: $viewModel.navigationPath) {
            RegistrationView()
                .environmentObject(viewModel)
                
                .navigationDestination(for: RegistrationScreen.self) { screen in
                    switch screen {
                    case .photoSelection:
                        ProfilePhotoSelectorView()
                    }
                }
        }
    }
}

class AuthViewModel: ObservableObject {
    // ...

    // A new enum that defines the various types of possible views in the navigation stack
    enum RegistrationScreen: Hashable {
        case photoSelection
    }

    // The navigation path
    @Published var navigationPath: [RegistrationScreen] = []

    // Example usages of the navigation path. These functions show how to programmatically control the navigation stack
    func showPhotoSelectionScreen() {
        self.navigationPath.append(.photoSelection)
    }

    func goToRootOfNavigation() {
        self.navigationPath = []
    }
}

如果我没理解错你的问题，你想使用NavigationStack，但它不适合你。
这里有许多遗漏的部分，但这里是我尝试使用NavigationStack来触发目的地，假定viewModel.didAuthenticateUser发生了变化。

struct ContentView: View {
    @StateObject var viewModel = AuthViewModel()
    
    var body: some View {
        NavigationStack(path: $viewModel.didAuthenticateUser) {  // <-- here
            RegistrationView()
                .environmentObject(viewModel)
        }
    }
}

struct RegistrationView: View {
    @State private var email = ""
    @State private var username = ""
    @State private var fullName = ""
    @State private var password = ""
    @State private var isVerified = false
    
    @Environment(\.presentationMode) var presentationMode
    @EnvironmentObject var viewModel: AuthViewModel
    
    var body: some View {
        VStack{
            AuthHeaderView(title1: "Get Started.", title2: "Create Your Account.")
            VStack(spacing: 40) {
                CustomInputFields(imageName: "envelope", placeholderText: "Email", isSecureField: false, text: $email)

                CustomInputFields(imageName: "person", placeholderText: "Username", isSecureField: false, text: $username)

                CustomInputFields(imageName: "person", placeholderText: "Full Name", isSecureField: false, text: $fullName)

                CustomInputFields(imageName: "lock", placeholderText: "Password", isSecureField: true, text: $password)
            }
            .padding(32)

            Button {
                viewModel.register(withEmail: email, password: password, fullname: fullName, username: username, isVerified: isVerified)
            } label: {
                Text("Sign Up")
                    .font(.headline)
                    .foregroundColor(.white)
                    .frame(width: 340, height: 50)
                    .background(Color("AppGreen"))
                    .clipShape(Capsule())
                    .padding()
            }
            .shadow(color: .gray.opacity(0.5), radius: 10, x:0, y:0)
            
            Spacer()
            
            Button {
                presentationMode.wrappedValue.dismiss()
            } label: {
                HStack {
                    Text("Already Have And Account?")
                        .font(.caption)
                    
                    Text("Sign In")
                        .font(.footnote)
                        .fontWeight(.semibold)
                }
            }
            .padding(.bottom, 32)
            .foregroundColor(Color("AppGreen"))
            
        }
        .navigationDestination(for: Bool.self) { _ in  // <-- here
            ProfilePhotoSelectorView()
        }
        .ignoresSafeArea()
        .preferredColorScheme(.dark)
    }
}

class AuthViewModel: ObservableObject {
    @Published var userSession: Firebase.User?
    @Published var didAuthenticateUser: [Bool] = [] // <-- here
    
    init() {
        self.userSession = Auth.auth().currentUser
        
        print("DEBUG: User session is \(String(describing: self.userSession?.uid))")
    }
    
    func login(withEmail email: String, password: String){
        Auth.auth().signIn(withEmail: email, password: password) { result, error in
            if let error = error {
                print("DEBUG: Failed to sign in with error\(error.localizedDescription)")
                return
            }

            guard let user = result?.user else { return }
            self.userSession = user
            print("Did log user in")
        }
    }
    
    func register(withEmail email: String, password: String, fullname: String, username: String, isVerified: Bool){

        Auth.auth().createUser(withEmail: email, password: password) { result, error in
            if let error = error {
                print("DEBUG: Failed to register with error\(error.localizedDescription)")
                return
            }

            guard let user = result?.user else { return }

            print("DEBUG: Registerd User Succesfully")

            let data = ["email": email, "username" :username.lowercased(), "fullname": fullname, "isVerified": isVerified, "uid": user.uid]

            Firestore.firestore().collection("users")
                .document(user.uid)
                .setData(data) { _ in
                    self.didAuthenticateUser = [true]  // <-- here
                }
        }
    }
    
    func signOut() {
        userSession = nil
        try? Auth.auth().signOut()
    }
}

struct ProfilePhotoSelectorView: View {
    var body: some View {
        VStack {
            AuthHeaderView(title1: "Account Creation:", title2: "Add A Profile Picture")

            Button {
                print("Pick Photo Here")
            } label: {
                VStack{
                    Image(systemName: "globe")
                        .resizable()
                        .renderingMode(.template)
                        .frame(width: 180, height: 180)
                        .scaledToFill()
                        .padding(.top, 44)
                        .foregroundColor(Color("AppGreen"))
                    
                    Text("Tap To Add Photo")
                        .font(.title3).bold()
                        .padding(.top, 10)
                        .foregroundColor(Color("AppGreen"))
                }
            }
            Spacer()
        }
        .ignoresSafeArea()
        .preferredColorScheme(.dark)
    }
}

除非将链接标记为isDetail(false)或使用.navigationStyle(.stack)，否则只能有一个NavigationLink级别。
这是因为在横向模式下，它使用拆分视图，并且链接取代了右侧的大细节窗格。