A model-driven telemetry pipeline streams interface statistics from an IOS-XE router to a gRPC collector. Operators report that the collector receives data for 'in-octets' and 'out-octets' but the 'oper-status' leaf that alerting depends on is always absent from every message. The subscription uses on-change reporting and the encoding is negotiated successfully. A packet capture confirms telemetry frames are arriving with valid payloads. What is the MOST likely cause of the missing leaf?