In this scenario the SNMP Poll Rate for a device's interfaces will be set to "true-null" if the network interfaces monitoring profile, or some variation of that profile, was associated to the collection that owned the device, and then it was subsequently dis-associated with the collection. As a result, the interfaces will be there but they will all be set to inactive and not be polled. To resolve, simply add the device(s) back into a collection that we know the network interfaces monitoring profile is applied to. Or, you can put them in a new collection and apply the network interfaces monitoring profile. You will then see the SNMP Poll Rate change from "true-null" back to whatever the poll rate is for the device