Support is telling me that this is normal behaviour and offers the following explanation
"As it is the Master which reports to vCenter, until a new Master has been established, the view in vCenter will not be accurate when it is isolated. (I believe vCenter checks the status of new Master/Slave elections every 2 minutes by default.) That said, when the Master is not isolated and still communicating with VC, it will give a more accurate view of the underlying VMs since it can still communicate with the Slave hosts over the storage heartbeats if available"
Cheers