Update: I've bought myself another CPU, which was W-2133, and it solved the issue, the stress test passes correctly now. Apparently the other CPU is degraded, which would make sense as the issue is in FIVR, which is on the CPU itself.
The issue is now solved.
It doesn't drop below 12v, not noticeably anyway. It could be that some of the peaks from the voltage ripple could dip for 50mv below the 12v for a fraction of a second, otherwise it's fine.