Asrock B550M Steel Legend wont POST after shutdown

Did you upgrade the PSU? From a quick google search it seems the stock one is only
500w.
That could easily be the issue.

Major OEMs like Dell, HP, and Lenovo do weird things with their power supply connections. In fact, I think there was even a case where Dell used the same connector with different pinouts, leading to catastrophic failures when their power supply was used with a standard motherboard.

I know for sure there is a converter cable available for standard power supply to Lenovo motherboard, because I own one. But I don't have any personal experience with Dell or HP.

Having wrote all that, quick search says HP OMEN 30L uses standard components.
So maybe the issue is something simpler, like incorrect wiring of the front-panel connectors. You know, all those non-standardized power switch, power LED, reset switch, HDD LED type connectors.

It's almost comical that they haven't standardized these connectors all these decades.

Oh I think I understand where we got confused about the power button. When I said it doesn't respond to the power button, I meant when its in that failed-post state it didn't seem to work to hold the power button down to turn it back off. The power button works fine when the system is operating normally.

And it turns out I was wrong about that anyways, holding the power button can turn it back off, I just wasn't holding it long enough.

If I hold the power button to turn it back off while its not POSTing, and then press the power button to turn it on again it still fails to POST. I must unplug the power from the PSU and hold the power button till the led lights go off (draining the power stored in caps). Then when I re-plug the power and press the power button again it POSTs successfully and starts up just fine.

That's whats so weird about it, its not like something obvious mis-plugged or whatever because when it works, it works. Its a real shame because this motherboard is the only one I could find with all the features I was looking for (m.2 wifi card slot, usb-c on the back panel, headers in reasonable locations for my case, etc.) But if I cant figure this out before my Amazon return window closes in march I'm just gonna have to get rid of it.

Is there any way to get real POST codes out of it instead of just those four useless red leds it has?

OK so I've done some testing and I think I've narrowed it down to ram. While I still had my previous HP mobo I had upgraded the ram, and I put that old ram into this system and it seems to post fine every time. Now I'm pretty sure that the new ram isn't bad because it worked in the old HP mobo just fine. So my guess is that this Asrock mobo is just extremely picky about ram timings. Though I can't be 100% conclusive yet because the issue is somewhat intermittent and only affects warm bootups.