Hi, @M4T
I assume it was created in a manual desktop pool with a full clone.
In such an environment, if a virtual machine is deleted from disk, the virtual machine will be completely deleted in vCenter Server and will maintain the status of being deleted unless Horizon receives a task completion notification.
So, is there any chance that the deletion took a long time in vCenter Server?
Also, did you finally remove the virtual machine name from the Horizon virtual machine tab without doing anything?