I ran into the same problem with a brand new Samsung and Apple TV 4k. I tried everything suggested here including multiple networks (2 wired and one wireless), disabling HDMI CEC, disabling AirPlay, etc.
Nothing worked. The Apple TV always wakes up a few seconds after putting it to sleep.
I have found a work around though. What you have to do is hold TV button on the Apple TV remote until the Sleep/Cancel screen comes up. Then using the Samsung remote switch to another input (like Live TV). Next just click the Apple TV remote touch pad area. This selects Sleep even though you can't see the options on the screen any more.
This works all the time for me. It's an irritating bug, but it's better than leaving the Apple TV running all the time.
I have another Apple TV (pre 4k model) with a Visio on the same wired network with default settings that never has this problem. I'm convinced that this is some Samsung/Apple compatibility issue. They should just communicate with each other and fix it.
Note that I left HDMI CEC on the Apple TV disabled, in part because it's a little irritating. I'm not sure if the work around works if you leave it on.