Same problem. I have a TV downstairs that “magically” is left turned on and I was tell Google to turn it off and would. Now, Google says it’s doing it but the TV never goes off. Now upstairs TV want turn on or off. Frustrating because it was very convenient when it worked.
HomeKit support seems to be reliable for power, if you have any Apple devices such as an iPhone, iPad, HomePod, etc. I used it for a while until I disabled HDMI CEC on my setup for other reasons.
I haven't been following this thread in so long, I forgot I had subscribed to it. I quit using Roku integration with Google, and Alexa because, for Google, I find Android TV works much better, and has more functionality in what you can do. Maybe Roku has updated their smart home integration, and it's better now. However, haven't tried it recently.