I believe this was fixed in 12.1 RU2 MP1 per the release notes fix here:
Clients report to the Symantec Endpoint Protection Manager as offline, even though they are online
Fix ID: 3002170
Symptom: Clients will randomly report into the Symantec Endpoint Protection Manager as offline, even though they are actively online and available.
Solution: Updated the client USN management to properly update the client status in the Symantec Endpoint Protection Manager reports.
I would suggest upgrading to the latest version 12.1 RU3