Hi,
I have a webmap which I use in a mobile app with Xamarin Forms and ArcGIS .NET Runtime 100.5 (problem also spotted in 100.4). I use a vector basemap in this webmap : World Navigation (https://www.arcgis.com/home/item.html?id=63c47b7177f946b49902c24129b87252) and the style in not honored on iOS, randomly on Android.
WebMap :

iOS screenshot :

iOS screenshot if I force the basemap in the code :
private async void OnMapLoaded(object sender, EventArgs eventArgs)
{
_mapView.Map.Basemap = new Basemap(new ArcGISVectorTiledLayer(new Uri("https://www.arcgis.com/home/item.html?id=63c47b7177f946b49902c24129b87252")));
}

So the basemap style is not honored when loaded from a webmap, but is OK when forced by setting the portal item id.
Any clue?
Here's how I initialize the map using a webmap item id:
var map = new EsriMap(new Uri("https://www.arcgis.com/home/item.html?id=5afc090637d54085a59a38d6f2f08adc"))
{
MinScale = _config.Map.MinScale
};
mapView.Map = map;