This issue is resolved in VMware vSAN Witness Node 7.0 Update 3L and VMware vSAN Witness Node 8.0b.
Workaround:
Following deployment of a Witness Appliance using 7.0 U3c (build: 19193900) or 8.0 GA (build: 20513097) OVA, the root password should be changed prior to rebooting or restarting OS of the Witness Appliance VM, this new password will persist across reboots.
If a Witness Appliance has already been rebooted and cannot be logged in with root password, a new replacement Witness Appliance should be deployed and the above applied.
Should deploying a new Witness Appliance not be possible or infeasible, please open a Support Request with VMware referencing this KB article.