Well, are both systems connected in the same way? Is your ATV4K connected directly to the TV or via the soundbar? All the HDMI devices need to be in a mode to support the CEC (and I've seen some soundbars that need to be told to play along). I've seen cases where ATV says "everybody off" but one of the connected components turns the dern TV on again immediately.
I assume you're using the ATV4K remote's power button with a long-press. I'm not certain if the "Sleep all connected devices" option in the ATV's Control Center does the same thing... but see what that does.
Anyway, I feel like I'm pulling info from a reluctant source. Was your main purpose to tell all of us that your Series 8 just doesn't work like your Series 7 and the Apple TV 4K is to blame because it's setup just like the other one that works? Fun experiment: swap the two ATV4K units and see if the problem moves.