It's a bit of a brute force approach but you could download the current WebMap definition through rest and then use the Map.FromJson to load it. That way you always load what is on the server.
Something along these lines would get you the WebMap definition
var handler = new ArcGISHttpClientHandler {ArcGISCredential = _portal.Credential};
string webMapJson;
using (HttpClient portalClient = new HttpClient(handler))
{
Uri requestUri = new Uri($"{_portal.Uri.AbsoluteUri}/sharing
webMapJson = await portalClient.GetStringAsync(requestUri);
if ( string.IsNullOrEmpty(webMapJson) )
{
Log.Warn("Unable to retrieve WebMap data");
return;
}
}
From here you should be able to just do
var map = Map.FromJson(webMapJson)
Thanks,
-Joe