Thanks for bringing this to the team’s attention. The problem arises from ArcGIS Online’s multi-factor authentication module. The issue has been fixed in ArcGIS Online, and the workflow can now be retested.
Although the issue is addressed in ArcGIS Online, for Native Maps SDK, it is recommended to use WebView2 for OAuth workflows.
The WPF OAuth public sample has been updated to use WebView2. Please refer to the updated code available at https://github.com/Esri/arcgis-maps-sdk-dotnet-samples/blob/main/src/WPF/WPF.Viewer/Helpers/ArcGISLoginPrompt.cs.