My guess? This likely means that the monitor never saw the sleep request and it is simply reacting to the output being shut down by the NUC.
I gave up on their monitor sleep support a long time ago. The latest 7th gen NUCs have the same monitor sleep issues that the 4th gen NUCs had. They blame the monitor every time. Baloney! I simply disable Monitor Sleep; it wastes a bit of power, but at least it works as it should.
Reinstall Windows and change HDMI cable - no change.
Monitor sleeps for 1 second and turn on the black screen (3s check hdmi signal) and again.
Same monitor, another computer, everything ok. Problem only with NUC.
I tested linux (Mint 18.3). Same Problem. Monitor can not sleep. :/