There are two ways for NSO to check whether a device is in sync or not. You are seeing the interplay between them.
- check-sync is a very fast mechanism. It basically reads a time-stamp of last change, transaction-id or some similar small piece of information to see if it is the same as earlier. If not the same, check-sync will flag an out-of-sync situation. That's what triggered the out-of-sync situation you saw.
- compare-config is a slower mechanism. It reads the entire config that NSO cares about, and compares everything to what NSO has in the database, and displays any diffs. If no diffs are found, this operation resets the out-of-sync flag.
That's what happened to you; check-sync detected a local change, and flagged out-of-sync. Compare-config checked everything that NSO cares about (perhaps only the odm YANG module), and saw that everything is in sync in that part, so cleared the out-of-sync flag. So this pattern will repeat whenever changes are made locally on the device outside the area of the YANGs included in the NED.
The root cause of this effect is again the lack of YANG coverage in your NED.
