In REM615/ REF615/RET615, SNTP IP is not holding, Once the pcmi file is re-written into the relay?
By default, writing the complete configuration to 615 device from PCM600 don't write the parameters that are locked in the Parameter Setting. In this case PCM600 tries to sustain the IP addresses that are in the device or to use the IP addresses from the system configuration (SCL).
Starting from PCM600 2.8, there is an option in the "Write to IED" -tool where you can choose to write also the locked parameters. This will guarantee that the IP addresses are used from the pcmi file.