I have an inkling this might be something to do with the AD synching, as I have just tried to add the user in question to a device and it has displayed an unexpected error.
This is consistent with some errors I have had in the past when AD synching has been playing up.
I will report back, but if anyone has any ideas in the meantime I'd still be happy to hear them
OK, please disregard my comments relating to old users.
The issue now seems to be that any user created on our AD this week, is disappearing from the device or group that I've added them to. It has happened several times today already.
I have stopped all scheduled tasks from running (including the AD sync) to see if this would stop the users from disappearing but it's still happening. I cannot work out what is prompting this to happen. I've also rebooted the server itself.
They show up when I search for users, they show up in the list of users on the device/group once I've added them, I've even successfully logged into the device as them. But at some undetermined point, they vanish.
I have a feeling I might have to raise this with McAfee support, as this is a huge problem