They set their machines to perform a reboot. The interval doesn't appear to matter. The symptom is that by the morning, some of the machines are still shut down. They must be manually powered back on and then everything operates normally. It will continue to occur and the machines that are shut down are never the same.
The reason this occurs is Citrix has an internal time-out where if an action doesn't occur, then it stops the action. When you set a Citrix Delivery Group to reboot, Citrix sends a signal to the Hosting Connection to perform a shutdown. Once, the hosting connection sends the signal that the machine has been shut down, then Citrix sends another signal to power the machine back on. If the shutdown signal is not received before the time out, then Citrix never sends another signal to turn the machine back on.
To resolve this issue:
Login to the Citrix Cloud Connector (it already has the Citrix Cloud PowerShell SDK installed)
Run asnp Citrix*
Run Get-XDConnection (login with your Citrix Cloud credentials)
Select the appropriate customer account if connected to multiple accounts
Run the following commands:
Set-BrokerServiceConfigurationData 'HostingManagement.MaxRegistrationDelayMin’ –SettingValue 60
Set-BrokerServiceConfigurationData 'RebootSchedule.MaxShutdownDelayMin’ –SettingValue 50
Reference article: https://support.citrix.com/article/CTX272494