Power Supply HDPlex GAN 250 cannot wake up from sleep

[tcf621]

Dear all, thx ahead. I am using the aforementioned PSU with
CPU: AMD Ryzen 5 5600G
MB: ASRock B550M-ITX/ac

Whenever the computer wents to sleep, it won't wake up on mouse / keyboard / power button
Power LED flickers
Indeed, when I click shutdown => similar happens

Things tried:
disabled hibernation
disabled fastboot
use Linux Live USB => same

Not sure if you guys have similar problem, and what can possibly help
Thank you very much