Symantec Endpoint Protection (SEP) clients have been deployed using system image. Sylink.xml file was changed before deployment on the machines, in order to contains appropriate PreferedGroup value.
Once they connect to Symantec Endpoint Protection Manager (SEPM) for the first time, they should receive a new Management Server List/Sylink.xml file, which contains less/more/different data about SEPM.
However, you notice there is no connectivity issue with SEPM but Sylink.xml file is not updated locally on the client.
Sylink.xml was manually modified.
Replace Sylink.xml file by a proper one, exported from the SEPM and/or delete Sylink registry keys: