Could you, please, open a support ticket and send us a sample test application demonstrating this issue, as we haven't been able to reproduce it in our local tests. The StackedBars appear as expected in our latest demos as well, you can find an example here
Perhaps we are missing some of the specifics of your scenario so it would be very helpful for us if we could have a look at your code and debug it.
All the best,
the Telerik team