I could really use some help from someone who knows USB, HID, keyboards, the intricacies of PCs interacting with keyboards at POST and such.
I have a Raspberry Pi Zero emulating a USB keyboard. It works great. However if I connect it via my KVM, two of my older PCs will not complete POST. Unplugging the Pi from the KVM's keyboard port solves this. Connecting the Pi directly to the PC solves this. No real keyboard has this issue.
Code and more details: https://gist.github.com/QBFreak/b7bc6a7421c2df8da63a43324c25e372