I did not see BIOS the chip select (SPI_CS0#) go low at any time so it looks like there was no activity toward the BIOS. It appears there was a crash log (FLOG) written to the ME region though. I will try to analyze it later.

While the board was on standby power, I was doing some voltage checking and a very strange thing happened. When I probed the capacitor on the RTCX1 side of the oscillator circuit (see below), it caused SLP_S3# to go high and the PSU came on. I was able to repeat it several times. I have no explanation for why this happens.

The PCH appears to have...

I'm working on an ASUS Prime Z390-A that isn't reacting to the power button. All of the standby voltages and signals are good. The PCH is receiving O_PWRBTN#, but it is not de-asserting SLP_S3#, SLP_S4#, or SLP_S5#. I started probing around and noticed the voltage on RTCX2 is around 1.5V, which seems crazy high. On RTCX1 it is around 300mV, which is normal. I don't see any waveform either. When the PCH is not powered, the resistance on RTCX1 is about 2.5M ohms and on RTCX2 it's about 8.5M ohms. When the PCH is powered on, RTCX1 shows about 7M ohms and RTCX2 shows OL.

I suspect the...

Yeah, I don't think it has anything to do with BIOS. I was trying to look at the VIDSCLK and VIDSOUT but I don't see any waveform for those. I tried with multiple known good CPUs, so the issue is in the board somewhere.