I have the ROG STRIX Z270F GAMING and from the first time that i install it i saw that the led from the power button in my tower was flashing when my computer was on. I did not pay attention because the tower led was very little.
Now that i changed tower i notice the same thing. To be honest, it is a little bit annoying because in the new case (Coolermaster H500p mesh) the power led big enough.I plug a lot of times the Pled cables but nothing happen. I change and position in the 3-pin position but nothing happen.
Power button flashing/blinking due to SATA or M.2 drive activity, user manual (English) Chapter 1, page 1-20, Hard disk drive activity LED (2-pin HDD_LED). Sometimes it could be a separate LED or combined with the power button LED.
If it is annoying, do not connect HDD_ LED system panel connector.