Thanks for reporting this. We've got an open issue on the backlog to deal with MapView resizing while rendering is active. I'll link this thread to that issue and we'll try to get it fixed in an upcoming release. For now, you'll have to wait for the map to be done drawing before resizing the MapView. Sorry about the inconvenience.
Thank you for your attention to this. We are able to workaround the problem for our present product release, but thought you should know about it. We look forward to receiving future updates.
This should be resolved in the recent v100.2 release.
Please download the latest Visual Studio extension from ArcGIS for Developers or update your NuGet packages via the NuGet package manager.