Not sure if you found this yet, as the post is a couple of months old. I just did some testing and here is what I found.
When the vCenter server boots (or I assume a restart of the services), if it can't find the user or group that is assigned to the Administrator role, it will delete that user/group and grant the local Administrators group for the windows server the Administrators role.
I found an entery in the vpxd-nnn.log:
[YYYY-MM-DD HH:MM:SS.XXX 01312 info 'Libs'] [ADS] Failed to lookup account DOMAIN\Account (err: 1332, [16,256])
[YYYY-MM-DD HH:MM:SS.XXX 01312 error 'App'] Removing invalid permission 19: user DOMAIN\Account not found
[YYYY-MM-DD HH:MM:SS.XXX 01312 warning 'App'] Removing permission for entity "group-X", group "DOMAIN\Account", role -1. Reason: User or group not found.
I found some other log entries that also looked like they identified the issue, but it was in the log both when my domain group didn't exist, and when it did:
[YYYY-MM-DD HH:MM:SS.XXX 01312 info 'App'] [GetLdapAdmin] No admin user set. Checking if 'Administrators' is part of LDAP admin list
[YYYY-MM-DD HH:MM:SS.XXX 01312 info 'App'] [VpxdLdap] Successfully retrieved LDAP admin principal.
There was also an entry in the Windows Application Log raising the event that my group was removed:
Log Name: Application
Source: VMWare VirtualCenter Server
Date: M/DD/YYYY H:MM:SS PM
Event ID: 1000
Task Category: None
Level: Warning
Keywords: Classic
User: N/A
Computer: [vCenter Server]
Description: Removing permission for entity "group-X", group "DOMAIN\Account", role -1. Reason: User or group not found.