Do you know which BIOS/microcode you were running before updating to 0x129? Multicore workload should never hit anywhere near 1.55V which is the cap from 0x129, and 13700K should be capable of doing 5.4GHz at 1.3V even with the worst bin, so it's a bit strange to see this 20% performance hit on CB.

If this is compared to <= 0x123 (before the introduction of "Intel Default Settings") then this drop may be attributed to low AC Load Line configuration in previous configuration, which essentially undervolting the CPU out-of-the-box. >= 0x125 BIOS usually bumped this up/lower this down to Intel's recommendation. It could be very possible that AC_LL is setting too high on the MSI board after the fix.

It's still very strange though, even if AC_LL is the culprit, that 253W is running 20% slower than 125W. For what it's worth, I have two systems running 13900K and 13700K both with ASRock board, and I see no difference in MT performance with 1-2% drop in ST (though 13700K is running with PL1=PL2=125W, but that shouldn't affect ST).

Perhaps by increasing the power limit to 250W you're also experiencing some thermal throttling now as well if the cooler can't burst fast enough or keep up with that higher wattage.

Its not getting above 75C its being cooled by a 360mm radiator. No thermal throttling at all.