I've managed to replicate the undesired behavior and have logged a new bug report
in our feedback portal regarding it. You can subscribe to the item in order to get notified once the development team starts working on a fix. As a thank you for your help in finding this issue, I've awarded you with some Telerik points.
For the time being, in order to solve this discrepancy, you should call the RadRibbonView's OnApplyTemplate()
method after adding the new tabs so that it is redrawn correctly.
I hope that you find such a workaround applicable.