The reason is in the way Cisco reports the Stackport status. The ifTable in the MIB data of the Cisco devices shows that the StackPort interfaces are stacked underneath the StackSub interfaces.
However, when the device is down the StackPort interface is removed from this table. This causes the stack port interface models to go with a status of unknown/not present depending on if a reconfiguration has occurred. But in either state, the PortStack interface model is grey.
But the StackSub Interface model does go operationally down (red). For this to change, Cisco would have to change the way it reports the StackPort status in the MIB data.
This is why we advise to turn on PollPortStatus to true for the StackSub models so that the alarm is at least raised on the Stacksub.