Some TV models do not enable the power off commands as part of their CEC protocol instruction set and so the only method to power off the TV is the TV remotes infrared.
I have this issue with an old LG TV from 2015, but I have found a workaround, which has been to add the Logitech Harmony Hub to the room and setup that to power off the TV using its own built-in infrared blaster, which can be actioned either via Amazon Alexa, or the Google voice assistant. The Hub is no longer sold by Logitech, but long may it continue to work - it has many other advantages too, such as switching TV channels, launching TV Apps etc.