Some devices showing unreachable status are not generating new alarms or events, but a CRITICAL "Contact Lost" alarm remains active despite it being raised a while ago.
DX NetOps Spectrum all releases
If the device is uncontactable for some time, what happens when you put it into Maintenance is that it goes Brown (Maintenance mode), but the CRITICAL alarm has not been cleared since it's still uncontactable. As you can see from the events, Spectrum tries and failes to contact the device:
When a Device is uncontactable, the CRITICAL alarm overrides all others, so no alert will be raised when the device comes out of maintenance due to the CRITICAL alarm still being active.