I have something similar that I've tried: shutdown from Windows 7 x64, unplug from power socket, press power button(to drain capacitors), then it briefly seems to power on from whatever charge it had left but shutsoff because it is still unplugged 🙂 After this, if I plug it in the socket, it starts to power on as if I had pressed the power button, but I didn't ! this on ASUS Prime Z370-A with latest BIOS Version 2401 2019/07/24
My setting was on Power Off, so it clearly wasn't respected.