That's the thing about metrics: they tell you stuff. This metric seems to be telling you that your vendor's MIB is broken. Take the evidence to your vendor.
I'd suggest showing them the raw metrics, without rate() or any other processing, so that they can see *exactly* what the device is returning.
If you call the API with a range vector query, you'll see the exact samples collected with their timestamps over that period:
curl -g 'localhost:9090/api/v1/query?query=ifHCOutOctets{ifAlias=~".*NNI.*",instance="10.85.0.82"}[10m]' | python3 -mjson.tool