Apple TV 4K – how to turn off device

That would be an issue with CEC. You can disable it through your TV and ATV settings if you wish, however if you want to continue using CEC then you will need to reach out to your TV manufacturer so they can investigate.

The Apple TV does not turn off, it goes to sleep, the only way to have it be off is to disconnect it from power.

No worries, and you're right that it doesn't hurt to try another configuration. If it works then that's great, however if you continue to have any issues just know that CEC is dependent on the host device and not the connected one (ATV) and each manufacturer has their own implementation. Unfortunately, the protocol has been a little hit or miss as a result and when issues arise the manufacturer would typically have to patch it from their end via firmware update.

Hello,

You can sleep Apple TV by pressing and holding the TV button on the Siri Remote > Sleep.