The IsDefault property works only on selectable items and it should indeed make sure that the default item is selected every time the Backstage is opened. I tested the property in a sample solution and I wasn't able to reproduce any issues.
Can you please have a look at the sample solution I attached and let me know if you can reproduce the issue on your side? Thank you in advance.
Yes it does work as advertised. I know this is going to sound odd, but I'm actually using the Backstage with another Ribbon product, so apparently the functionality is dependent on the Telerik Ribbon itself. I was able to mimik the behavior by trapping the tab switch event.