Stepping into async code sometimes doesn't work

I'm following the steps [here](https://github.com/OmniSharp/omnisharp-vscode/wiki/Debugging-into-the-.NET-Framework-itself) and it works except I can't step into async methods.

When I try to step into, for example, `await authService.SignInAsync();`, the next line the debuggers shows is something in AsyncMethodBuilder.cs. Then if I step into again it bumps me back out to where I tried to step into the async method.

I expected to see the `AuthenticationService.cs` source file.