The Witness host within vCenter cluster check in the vSAN Health Service verifies that the witness host is a standalone host, and not part of the vSAN-enabled cluster. If this health check is not green (OK), the stretched cluster has configuration issues that affect its operation.
If the witness host resides within a vSAN-enabled cluster, it causes problems with vSphere DRS, HA, and EVC. The witness host must not be a member of any vSAN-enabled vCenter cluster.