Hey there @max_mix! Welcome to the community! So let me preface this by saying to be careful maxing out your components when there’s shutdowns, you do not want to damage your components.
From the symptoms alone and the fact that you used OCCTs power test and it shuts down immediately (with no crash and no logs), this leans towards an issue with your PSU either not supplying enough power to keep everything powered at max load, or there being a fault somewhere between the PSU and the motherboard that is tripping it. In either situation the first move is to check your PSU’s limits and add up your other component’s load to see if you could be pushing it too hard. When I worked in IT I saw this rarely and the symptoms match up exactly.