I am not from Intel, but your situation sounds somewhat like my limited understanding of Intel Ready Mode (or Away mode?). I think Intel includes such software in their driver bundle, so maybe you enabled it. Sorry that I don't know more about that software but one of the FAQ says:
Intel® Ready Mode Technology does not prevent your computer from entering hibernation mode. Intel® Ready Mode Technology does prevent the system from entering sleep mode.
I have Win8.1, NUC 5i5RYH and it's not hooked up to a TV but a Dell U2414H single monitor. To get my unit to "sleep", I have to do it via Windows Start button right click> Shut down>Sleep. If I use the Power Plan in Windows>Balance>put the computer to sleep>30 minutes, It sometimes does and sometimes doesn't. When it does, and I tap the keyboard to wake it up, my screen is messed up with visual 'junk' so I just use the Windows Start button.method ...and it's been working every time without incident. Power button on NUC 'breathes' orange..sleeping until tapping on the screen brings it back to life (blue). Nothing set in then BIOS for hibernate. Running also latest BIOS and Graphics Driver. My setup isn't quite like yours but it could be a cable issue to the TV? Tried different HDMI cable?
You were on the right track! I disabled Ready Mode Technology and sleep started working. Cheers!