So these guys [1] mention something similar where HDMI from a TV is backfeeding 40-50 volts into a cable box. This could be because of many things from electrical outlet wiring to power supply issues on the monitor to a bad component on the monitor giving a high voltage, or the monitor is badly grounded, etc, etc.

I read the original thread but it doesn't look like you've measured the voltage at the HDMI port wrt motherboard ground. I think we're assuming it's 5 volts, but it could be higher, and it could have shorted (or weakened) a component on your motherboard. And that would explain why a 100 ohm resistor didn't give a meaningful voltage drop.

If you need an isolation solution, Amazon sells a 50ft fiber optic one way HDMI cable [2]. The thing I don't know is if there's any actual copper to provide power over the link. There are other options which transmit the HDMI signal over pure multimode fiber as well [3].

Or you can go with a DP KVM, since you're on L1T, they sell a few DP models. I have one I purchased from L1T, and I like it a lot.

Definitely though I would check out the outlets to make sure they were wired correctly. Incorrectly wired outlets because someone tried to DIY it in the US is absolutely a problem.