Custom reboot dialog box not appearing for Apple Updates

I am seeing an issue when using a policy to force applying of Apple updates from the Apple Update server. IF a restart is required the policy is set to display a custom dialog box letting users know that a reboot is required to apply patches, but instead a default dialog box is being displayed "This computer will restart in 5 minutes. Please save anything you are working on and log out by choosing Log Out from the bottom of the Apple menu."

Seeing this on both Mojave and High Sierra mac devices - running JAMF Pro 10.13 on prem. Wondering if anyone else has seen this?