Users added using es-config -u <username> and es-config -l <username> -M <modulename> -z snmpusercategory are removed automatically when the agent is restarted. This problem occurs for modules such as Kernel Reader Simple and Agent Statistics.
Workaround: Add the users manually by adding the slice:object=obj_value entry in the modulename.dat file present in /var/opt/SUNWsymon/cfg directory.