Tags associated with a Virtual Machine are lost when the VM is unregistered from one vCenter Server and registered to a different vCenter Server which are not in Enhanced Linked Mode.
VMware vCenter Server
This is an expected behavior when a server is unregistered and then registered into a new or different vCenter Server loss of tags occurs because vSphere Tags are metadata objects stored and managed by the vCenter Server database and not by the virtual machine configuration file itself.
Need to create vSphere Tags and assign to VM.
Reference document to create and manage Tags : What Are vSphere Tags and How to Use Them