It's a decent unit made by a reputable manufacturer. "High Power" is the brand name for Sirfa power supplies. They make power supplies for Zalman, Rosewill, Silverstone, Silentium, etc.
This particular unit seems to be a cost down version of the GP600B-HP EVO. I haven't seen a tear down of it, but it looks like they swapped the fan out for a cheaper sleeve bearing fan and it may not have Japanese brand caps like the EVO.
RTX 3090 has transient load spikes and could trigger your PSU protection shutting down the system. It happened a lot even with known and good 850 watts PSU. Seasonics even said that in their lab they saw spikes to 550 watts on the RTX 3090.
Read the post below for cool information.
View: https://www.reddit.com/r/hardware/comments/k5lgm4/psa_3090_3080_transient_load_spikes_north_of_500w/
This is not the issue tho. The system would still POST and the shutdown would happen at load.
Recorded peak power draw of a 11900K is 332 Watts (Oced). Add that to the RTX 3090 and the spikes + the rest of your system and you can see that you have an issue here.
Did you test the second PCIe slot?
Test with the 750 watts too. Just to see if it's a faulty 800 watts PSU.