Symptoms:
When attempting to select a witness host during vSAN stretch cluster configuration you get error "The witness host must not be part of a vSAN enabled cluster"
The witness is not in use by another cluster per vCenter and is not a Shared Witness
Running esxcli vsan cluster get on the witness shows it is in a cluster with no other hosts.
The witness has the vSAN service enabled already but is not joined to a cluster