Report it to Microsoft. Hopefully you noticed the "Beta" tag on the program you're using, which means it's incomplete, and may contain bugs. Well, more so than your average Microsoft program anyway.
Report it to Microsoft so the developers can try to duplicate the issue and resolve it for the final build.
I have a problem with ie8 beta 2 . When I want to replay e-mail with my yahoo e-maill basic. Mouse pointer will disappear in subject. It appear in message body and others places. What can I do? I change a new mouse it still is same.