The way I understand it Time Sync shares the internal clock of the master with the slave unit so they all run the same time, it does not set or change the time of the master.
You need to locate the source of time to the master (plc) it could be a GSV Statement in the logic, it could be an HMI or PC writing time to the plc.
The first steps I would take are as follows:
1) Check the program for GSV WallClockTime Instruction.
2) CHeck the web page of the Ethernet Card(s) and see what connects to this PLC that might be setting its time.