Desktop Computers

So I borrowed a spare AM4 motherboard and it's having the exact same issue, PC will randomly power off and won't turn on until I power the PSU off and try to turn the PC on to 'drain' it.

I'm going to find a spare PSU and try that soon.

I am wondering if it might be heat related as I noticed if I play Satisfactory for a few hours it will reliably pop the issue after 2-4 hours whereas I have left the PC on overnight without much running and it is still fine in the morning. Another hint is that often if I reset it immediately after the issue occurs it pops again. If I leave it to cool down for 60 seconds then it will cleanly boot. Not a huge sample size so far though.

I ran hwinfo and noticed the CPU is getting up near 90C running satisfactory and up to 95C running PoE which is pretty warm even for an AMD.

Maybe when it gets too hot something is expanding just enough to cause a short.

Yes, a few weeks back it got up to 98C running PoE and it was starting to have performance issues as I guess the CPU was probably stepping down to try to avoid cooking itself. PoE's low texture quality mechanism was kicking making everything pixellated and I was having some rubberbanding too, all I assume because of CPU. I dismantled the whole PC, used compressed air to clean everything out, cleaned and re-pasted the CPU cooler back on and it brought it down about 5C to start capping at ~93C and I stopped having the performance issues.
It's only in the last ~week that I started having this power issue. Maybe the CPU is slightly damaged due to overheating and when it gets hot it's shorting something.
If a replacement PSU doesn't solve the issue I'm just going to go ahead and upgrade to AM5 (cpu/mb/ram replacement), something I was hoping to hold off for ~2 more years since the performance of my 3700X is still more than enough for what I do.
I just really hope it's not my video card that's fucked since I only picked up my 3070Ti a year ago.

Still pretty bad temps. What does the rest of the case look like, cooler/airflow etc? I see a lot of builds that use way too much paste. Metal-to-metal contact is best for the cpu and cooler, the paste is only there to help fill the micron sized air pockets that exist because neither cpu nor cooler is perfectly flat. Put small dab on it and spread it around with a scraper, you want it to be like melted butter on warm toast, so thin that you can see through it.

Also turn off any bios-style CPU overclocking, for testing at least.

You have a 3700X? Hottest I could ever get mine under stock settings was about 64C with CB r23 multi, using a NH-F15. Stock PPT for that CPU is only 88w. Check your cooler mount - remove it and take pictures of the thermal paste impressions on the bottom of the cooler and top of the CPU. If the paste impressions looks good, reseat or clean/repaste, and in the BIOS try Loading Optimized Defaults. Enable DoCP, save, reboot. Check temps again.

I still have some data from my 3700x testing/tuning, if you want to check what voltages your CPU is pulling under load we can compare. Using HWiNFO64 the relevant value should be "CPU Core Voltage (SVI2 TFN)" for that CPU. For reference, at stock/defualt (which can vary by mobo brand and cpu lottery), I was getting 1.464v average across a 1 core r23 20 minute run, and 1.216v across multi-core r23 20 minutes. Non-PBO, non-OC, non-Mobo-shenanigns have the average voltage at 0.998 for 1c, and 0.988 at 16c r23 20m runs (this is base/non-boost behavior) and temps maxed at 50c.

Hottest I could get it was maybe 88C by adjusting the PPT, EDC, and TDC values, using PBO, and playing with Asus' poorly worded options, but that netted almost no single core performance gains and very limitd multi-core gains, since the CPU has hard power limits.

Replacing the PSU appears to have fixed my power issues. I've had Satisfactory running nonstop for 2 days now when previously around 4 hours was as long as the PC would go before shutting off.

CPU is a 3700X and yeah it's definitely too hot. The whole build is pretty clean inside now since I've dismantled it a bunch of times lately and cleaned it out. Did have too much dust in the fans and heatsink before I cleaned it out for the first time some weeks back. BIOS is all default, it's an MSI B550 MB. Previous one was an MSI X570 - which I could go back to using since it's now apparent it wasn't causing the issue.
I've never tried to OC it.