I created a test project to test the described behavior on my end. However, I was not able to observe the same behavior. When press a Tab key the focus moves through the buttons in a docked window as well as in a floating window. You can check the attached gif file.
I attached my project to this thread. Could you please refer to it and let me know how it differs from your set up. Is the issue reproducible on your end? If it is, it would be greatly appreciated if you can specify which Telerik version you use as well.
Looking forward to your reply.