After upgrading from 7.9 XenApp environment to 7.12 the delivery group restart schedule is no longer working if Notification Message is selected.  The event logs show that the Delivery Group started and the abandoned the job:

The Citrix Broker Service has terminated the processing of the 'GroupReboot' for 'DeliveryGroupName". The reboot cycle ended with the final state 'Abandoned'. Summary: '0' machines successfully rebooted, '0' machines failed to reboot, 'x' machines were not processed.

Complete the following steps:

1. Download the private LC6766. (Contact Citrix Support to request this private).
2. Connect to a test VDA 7.12 and stop the service Citrix Desktop Service.
3. Navigate to “C:\Program Files\Citrix\Virtual Desktop Agent”.
4. Rename the existing BrokerAgent.exe to BrokerAgent.old and copy the private version of BrokerAgent.exe into the folder.
5. Restart the Citrix Desktop Service.

Workaround:

Do not send any message while  reboot schedule.

Note: If we are sending the messages  then message will pop up  however no reboot to the machines where session is connected.

Update: This fix is now part of the 7.13 release of XenDesktop.