I just tested out the sleep mode using same Intel® NUC model and the fan stayed quiet when sending the computer to sleep. I did notice fan spinning when waking up the Intel® NUC.
I just made sure the Intel® NUC was currently running the latest BIOS 0042
Before updating to the latest BIOS, please try loading default settings by pressing F9, then F10 to save and exit and try again.
Hope this information is helpful.
Please do let me know if you need further assistance with your Intel product, I will be glad to assist you.
I have seen this issue as well. The KY NUC is not, in fact, completely shutting down (LED on power button remains lit). The monitors are turned off but the power stays on and the blower (it's not a fan!) slowly speeds up over time. This makes me think that the system is getting stick in a hard loop and this is preventing the unit from actually getting to the power-off (S5) transition.
I am running on the latest BIOS.
Does this issue still happen if you downgrade the BIOS to an earlier version?
You need the recovery method if you will test it with the BIOS version 0041