Comment 8 for bug 402795

That command also fixed it for me. I backed up the fresh configuration from the installer (grub.cfg.old) and compared it to the one after running 'sudo update-grub2' (grub.cfg.new) and the command seemed to have added my Windows partition the "os-proper section" as well as having it in the "otheros" section. I have attached the two cfg files.

So, the command definitely works as a workaround (should probably be posted on the Alpha 3 release notes), and the behavior could possibly be implemented into the installer.