Also on WS12 running in a Win10 host with a Win7 guest. My workaround was to RDP into the guest using RDTabs by Avian Waves.
Maximize the program when it starts up. Set up your RDP connection to the guest. Use the default Display/Screen size settings first - you can customize later. It will stretch the guest session to fill up the screen. It even has the option to scale down the screen if you set up a session screen size that is larger than the host screen size.