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.
However, on some motherboards, what is that?
Also an indication of:
https://www.techdim.com/what-is-hdd-led/