In Collector App a numeric field used as the unique identifier that is not editable is changing to 0 during the sync process on just some of the records. Why would this be happening?
Are you using hosted feature services on Portal/ArcGIS Online or are you using services from ArcGIS Server?
If server, which version of server? What kind of database are you storing your data?(sql, oracle, other?)
Is your data archived or versioned?
are you on the latest version of Collector?