Just stumbled upon the same problem even though years later. I got it to work by altering the XML file stored in home/osmc/.kodi/userdata/peripheral_data/.
Change <setting id="standby_devices" value="231" /> to <setting id="standby_devices" value="36037" /> and then reboot. 36037 seems to be the ID of the TV. After this change i can call the built in function ‘CECStandby’ and the TV turns off.
Found the solution here: CEC Control via JSON API