OK, this is "Working as Designed".

The "Unregister Device" action places the device into the "zDeletedDevice".
Devices listed here will not re-register to the zone except for an explicit "zac reg".

Just Delete the device instead of Unregistering from the "Actions" Menu in the ZCC.
That appears to be the only place left that may place devices there.

However....It's a very useful feature to help ensure a device does not come back automatically.....
I was stumped the other day on how to do this en-masse for another customer.

Note: I've made a ZCC Help doc bug to help clarify the details around that action.