You're using a preview product, so there are bound to still be bugs in it. After all, there are still plenty bugs in the non-preview versions!
You should report the problem to Microsoft so that they can investigate:
Visual Studio Feedback
The easiest way to start is by clicking the feedback button in the top-right corner of the Visual Studio window. This will let you attach internal log files, screen-shots, or even a video demonstrating the problem.