When the device is kept under maintenance mode the interfaces under the devices are not reflecting the maintenance mode.
By default when the device goes down the maintenance mode will not be propagated to the interfaces.
If we need to propagate the maintenance mode then we need to set rollMMAlarmToIF to Yes
Go to the attributes of the device
Set the attribute rollMMAlarmToIF to yes
Now this will propagate Maintenance Mode to the interfaces/ports.
Check the events on the port which will show the port under Maintenance Mode when you set the device under Maintenance Mode.