SwiftData relation not update UI

The 1 to 1 relation models:

@Model
class Country {
    var name: String
    var population: Population?

    init(name: String, population: Population? = nil) {
        self.name = name
        self.population = population
    }
}

@Model
class Population {
    var country: Country?
    var count: Int

    init(country: Country? = nil, count: Int) {
        self.country = country
        self.count = count
    }
}

I don't know why not update the UI when click population label. But if I expand the CountryDetail it will works.

struct DemoView: View {
    @Query var countries: [Country]
    @Environment(\.modelContext) private var context

    var body: some View {
        NavigationStack {
            List {
                ForEach(countries) { country in
                    VStack(alignment: .leading) {
                        Text(country.name)

                        if let population = country.population {
                            // bug: not update
                            CountryDetail(population: population)

                            // like above but expanded, it is ok
                            Text("population: \(population.count)")
                                .onTapGesture {
                                    Task.detached {
                                        try await DemoData(modelContainer: DemoData.sharedContainer).changePopulation()
                                    }
                                }
                        }
                    }
                }
            }.safeAreaInset(edge: .bottom) {
                Button("Add") {
                    let country = Country(name: "US")
                    context.insert(country)

                    let population = Population(country: country, count: 0)
                    country.population = population
                }
            }
        }
    }
}

struct CountryDetail: View {
    @Bindable var population: Population

    var body: some View {
        Text("population: \(population.count)")
            .onTapGesture {
                Task.detached {
                    try await DemoData(modelContainer: DemoData.sharedContainer).changePopulation()
                }
            }
    }
}

@ModelActor
actor DemoData {
    static let sharedContainer = try! ModelContainer(for: Country.self, configurations: .init(isStoredInMemoryOnly: true))

    func changePopulation() throws {
        let descriptor = FetchDescriptor<Population>()
        let all = try modelContext.fetch(descriptor).first

        if let population = try modelContext.fetch(descriptor).first {
            population.count += 1
            try modelContext.save()
            print("population count", population.count)
        } else {
            fatalError("not found population")
        }
    }
}