SwiftUI Tutorials: Map Preview does not show the location I enter

Hey guys, I was just learning the SwiftUI Tutorials, and I was quite confused about the Map preview in the following code:

import SwiftUI
import MapKit

struct MapView: View {
  var coordinate: CLLocationCoordinate2D
  @State private var region = MKCoordinateRegion()

  func setRegion(_ coordinate: CLLocationCoordinate2D) {
     region = MKCoordinateRegion(
      center: coordinate,
      span: MKCoordinateSpan(latitudeDelta: 0.2, longitudeDelta: 0.2)
    )
  }
   
  var body: some View {
      Map(coordinateRegion: $region)
        .onAppear {
          setRegion(coordinate)
      }
  }
}

struct MapView_Previews: PreviewProvider {
  static var previews: some View {
    MapView(coordinate: CLLocationCoordinate2D(latitude: 34.011_286, longitude: -116.166868))
  }
}

As you see, the preview shows a map of blue sea, instead of what I would like to see (the location of Joshua Tree).

However, when I use MapView() in another view structure by embedding it within a HStack (VStack, ScrollView works as well ), the preview shows perfectly what I want:

import MapKit

struct MapViewDemo: View {
  var body: some View {
    HStack {
      MapView(coordinate: CLLocationCoordinate2D(latitude: 34.011_286, longitude: -116.166868))
    }
  }
}

struct MapViewDemo_Previews: PreviewProvider {
  static var previews: some View {
    MapViewDemo()
  }
}

Could you help me understand why it works this way? Is it the particular way how Map or .onAppear works?