If a macOS device's Dock is refreshing/the screen is flickering every ~30 minutes, this may be due to settings in items that have been added to the policy. Catalog items that can be configured to affect the Dock are Dock MDM configurations and Self Service Configurations.
When a Self Service configuration is enforcing the MacManage icon to appear in the Dock, and a Dock MDM configuration is also being deployed, flickering behavior can occur.
When both a Dock configuration and a Self Service configuration (where the Dock Icon for MacManage is enforced) are being applied to a device, best practice is to:
Add the MacManage app to the Dock configuration as a Static App:
un-check the Show Icon in Dock box in Self Service
This will allow the MacManage icon to appear in the dock without being enforced by two separate mechanisms and should prevent the flickering behavior.