Thanks for sharing your issue in public. This might be beneficial to other users who come across a similar problem.
As explained in your support request, what you are experiencing is an erroneous behavior which is rooted in Apache Cordova.
On one hand, with Apache Cordova, orientation settings are not really respected for Windows Phone 8. When you set them in config.xml, they are ignored (this is what using the Project Properties in AppBuilder does - it sets the orientation preference in the config.xml for the corresponding platform). The way to correct this would be to manually set the orientation in the MainPage.xaml file.
Here is where the AppBuilder issue, or rather missing feature, comes in place. You need to manually edit MainPage.xaml to indicate the orientation restriction. And currently, there is no viable way to do that in AppBuilder. We have already logged this improvement in our feature tracking system but will appreciate it if you log it in our feedback portal
so that other users might vote for it.
Share what you think about AppBuilder and Telerik Platform with us in our feedback portal, so we can become even better!