MapKit offers showsTraffic: Bool, which is great for displaying live traffic data on the map. However, MapPolyline sits above it, which makes it quite useless. Is this the expected behaviour?
SwiftUI MapKit
Use the .mapOverlayLevel(level:) modifier on your MapContent to position polylines relative to other map elements. Setting .aboveRoads places polylines above roads but below labels and traffic, keeping traffic data visible. Setting .aboveLabels places them above everything including traffic.