The DataContext of the (Transitioning)ContentControl is bound to a class with a Enum property CardTypeValue which fires INotifyPropertyChanged. The ContentTemplate is set via DataTriggers (one for each Enum value). The default ContentControl recognizes the notification and changes its ContentTemplate. The TransitioningContentControl doesn't do this.
ContentControl :

TransitioningContentControl:

Additional info:

@seb30 I think I know what's the problem is, but it would be nice if you can separate this one in a simple app.
I'm out of office for a week. I will do this after my vacation.
@seb30 It would be nice if you can test the latest changes against your app after your vacation :-D
The latest v2.0 alpha NuGet package can be found here https://ci.appveyor.com/nuget/mahapps.metro
@punker76 I've tried with the latest 2.0.0-alpha0067 and … check the results by yourself:

Thank you!