UIMVM models placed in maintenance mode are being removed from maintenance mode automatically by an "Unknwon User"
The "SetMaintenanceModeToChildren" attribute was enabled on the ESX host. This attribute was introduced for a requirement where customers wanted to set all the children of an ESX host into maintenance by putting that ESX host model into maintenance thus saving them the effort of putting each VM in maintenance mode, generally for cases where there is a scheduled downtime or maintenance planned for that ESX host.
Now if a vm is migrated from this ESX host to another ESX host then that vm comes out of maintenance mode if the new parent ESX host is not in maintenance (in short, maintenance status is inherited from the parent ESX host if "SetMaintenanceModeToChildren" attribute is set to yes ).
If you don't want to see this behavior, Set the "SetMaintenanceModeToChildren" to false. So irrespective of the parent ESX's maintenance status, the VM will continue to be in maintenance until it is taken out of the maintenance mode by the user and until that happens all the alarms on the VM will continue to be suppressed.