The current sensor code always disables background scan for all master image and cloned VDI devices installed with the required VDI=1 and GROUP_NAME="Virtual Desktops" parameters ignoring the policy settings.
Carbon Black is planning to adjust the sensor code to honor the "Run background Scan" for the Master Image when it is enabled on the policy. However, the sensor code will continue to ignore the policy settings and keep background scan disabled for all cloned images. This KB will be updated with the sensor release that will contain this fix once it has been confirmed.
Cb Defense: How to deploy sensor to a non-persistent VDI Master Image?
Cb Defense: Background Scan FAQ