From the official documentation (https://docs.opennebula.org/5.4/operation/vm_management/vm_instances.html):
“Undeploy -> The VM is shut down. The VM disks are transfered to the system datastore. The VM can be resumed later.”
I understand that undeployed VMs still hold their “non-persistent” data. But as soon as you terminate a VM with non-persistent disks all changed data (to the base image) is lost.