Apple TV 4 does not stay on HDMI Input

I am using Latest Samsung 65" TV. I have 3 things connected to my TV through HDMI port, Apple TV 4, Chromecast, Set top Box. Somehow since I bought Apple tv 4 and connected to TV. Whenever I go to HDMI input of Apple TV it does not stay there and jump back to HDMI input of Set top box. So literally I cant use this devices together if I keep HDMI CEC on in my TV setting. HDMI-CEC suppose to jump to that HDMI Input when that device turned on.

This is what I have tried so far,

• I can use this devices together if I turn off the HDMI CEC in tv setting but in that case I dont get the functionality of automatic HDMI input change and turning on and off the tv by Apple tv remote does not work too.
• Take out the Apple tv HDMI cable from TV and other 2 devices works fine
• Take out the Set top box HDMI cable from TV and other 2 devices works fine.
• Tried changing the HDMI cables, Ports but none of them works.

So looks to be problem with Apple tv and Set top box working together.

I have tried to contact Apple support and he searched in his database for an issue and told me that Apple tv 4 does not support HDMI-CEC. Wow!!!!

When I search in google for Apple tv 4 and HDMI cec I get Apple's own documentation for this, specifying the words and its new feature. Now I dont know advisor was lying to get rid of the call or didnt have enough knowledge to address the issue but when he told me that Apple TV 4 does not support HDMI-CEC it does not make sense for sure.

Set top box company does not have any resources to examine the issue and it is blaming Apple tv.

Please help.