I am not from Intel and don't have particular explanation, but some things you could try:
Investigate WOL settings not just in BIOS but also in your network device drivers (both wired and wireless), for example in Windows it is something like Properties-Power Management tab (of each driver)
If you are using infrared remote in same room (such as for Philips TV or Xbox), consider disabling those new remote control settings in the BIOS (and/or placing some heavy cardboard/tape over the IR window near the front right of the NUC).
It is important to narrow down a little bit to understand if this behavior is caused by something within the operating system or by hardware.
As Dougho said there are options in the Power Management properties of the Wi-Fi and LAN adapter drivers that can wake up the system. Make sure these options are disabled just in case.
As per testing to gather more information please try the following.
- With any BIOS version, disconnect the LAN from any router or network. Shut down the system from Windows (S5) as you normally do and compare
- If problem persists, while having the LAN disconnected, remove the Wi-Fi card, if using one, and shut down the NUC from Windows.
- If the problem persists, remove the main Solid State Drive. Turn the NUC on and enter the BIOS by pressing F2 when posting. Run F9 to load defaults and then F10 to exit saving changes. Press down the power button for 10 seconds until the NUC is totally off and compare results.
If the NUC stills comes back up by itself while having the LAN disconnected, the Wi-Fi and SSD removed then you have a faulty NUC.