I'm using the latest alpha channel and try this:
Create new .NET Standard 2.0 Project
Add reference to Xamarin Forms
Add a couple of new Xaml content pages (one of them I turn into an App.xaml/App.Xaml.cs)
Notice that VS properly groups the Xaml and cs files together in the project structure
Close and reopen VS. Note that the Xaml and cs files are no longer grouped together, and that a duplicate of each of the .xaml files shows in the project tree.
Is this a known issue or am I doing something wrong?