Summary: If your Siri Remote power button intermittently fails to wake your Samsung TV (HDMI-CEC enabled), try the voice command "Turn on the TV" via a HomePod. The remote button communicates with the Apple TV over Bluetooth; the voice command reaches it over your LAN. In 90+ days the voice route has never failed for me.
The problem
Pressing the Siri Remote power button always wakes my Apple TV and Sonos Arc Ultra 5.1 system, but intermittently fails to turn on my Samsung S95F.
The workaround
Say "Turn on the TV" to a HomePod. It turns on everything: Apple TV, Samsung S95F, and Sonos Arc Ultra 5.1 system.
What's different between the two
• Voice command (LAN): The HomePod sends the request to the Apple TV over your network, and the Apple TV then sends the HDMI-CEC wake command to the TV.
• Siri Remote power button (Bluetooth): The remote tells the Apple TV over Bluetooth, and the Apple TV then sends the HDMI-CEC wake command to the TV. This path intermittently fails to wake the Samsung. I don't know what causes the intermittent failure.
Required settings
• Apple TV: Settings → Remotes and Devices → Control TVs and Receivers → ON
• Samsung TV: Settings → Connection → External Device Manager → Anynet+ (HDMI-CEC) → ON
• Audio: Sonos Arc Ultra connected to the TV's eARC HDMI port
• HomePod: Same network as the Apple TV, Siri enabled
My setup: Apple TV 4K (3rd gen, 128GB Wi-Fi + Ethernet, A2843, tvOS 27), HomePod (original) and HomePod minis (all HomePods audioOS 27), Samsung 4K OLED with current firmware.