When empty disk is added to running VM as Persistent Hotplug disk, it can't be made made Persistent disk by restarting the VM. 

1. Navigate to VM "Storage" page
2. Add empty disk "disk-blank" to VM. It has "Persistent Hotplug" label.
3. Click on kebab button and make disk-blank persistent.
4. Restart the VM

No warning is displayed (or displayed briefly and then disappears). The disk keeps "Persistent Hotplug" label. 

Warning "Pending changes" is displayed: Disks > Make persistent disk - (disk-blank) (see "4.17.png")
After reboot the "Persistent Hotplug" label is gone.

It worked differently in 4.17 (see "no warning.mp4")