MapKit Annotations Disappearing

The accepted answer from Leszek Szary is correct.

But there is some fineprint. Sometimes MKMarkerAnnotationViews are not rendered, even if

view.displayPriority = .required

is set.

What you are seeing is a combination of different rules.

1. MKAnnotationViews are rendered from top to bottom of the map. (It doesn't matter where north is).
2. If MapKit decides to draw overlapping MKAnnotationViews, then the MKAnnotationView nearer to the bottom is drawn on top (because it's drawn later)
3. Not only MKAnnotationViews, also titles rendered below MKMArkerAnnotationViews need space. The rendering of those titles is influenced by markerView.titleVisibility. If markerView.titleVisibility is set to .visible (instead of the default .adaptive), then this title is stronger than a MarkerAnnotationView that is rendered later, even if the later MarkerAnnotationView has a displayPriority = .required. The MarkerAnnotationView nearer to the bottom is not rendered.
4. This even happens if the MarkerAnnotationView nearer to the top has a low displayPriority. So a MarkerAnnotationView with low displayPriority and .titleVisibility = .visible can make a MarkerAnnotationView nearer to the bottom with displayPriority = .required disappear.

I am not aware of a documentation of this behaviour. This is the result of my experiments with iOS 12. My description is sipmplified.

NiltiakSivad's solution works but it reverts to the old iOS 10 look. If you want to keep the new iOS 11 balloon markers for iOS 11 and use the old pin look only for older iOS versions then you can implement the delegate method as below:

func mapView(_ mapView: MKMapView, viewFor annotation: MKAnnotation) -> MKAnnotationView? {
    let reuseIdentifier = "annotationView"
    var view = mapView.dequeueReusableAnnotationView(withIdentifier: reuseIdentifier)
    if #available(iOS 11.0, *) {
        if view == nil {
            view = MKMarkerAnnotationView(annotation: annotation, reuseIdentifier: reuseIdentifier)
        }
        view?.displayPriority = .required
    } else {
        if view == nil {
            view = MKPinAnnotationView(annotation: annotation, reuseIdentifier: reuseIdentifier)
        }
    }
    view?.annotation = annotation
    view?.canShowCallout = true
    return view
}