Intel NUC, no video output detected by monitor during boot, through KVM switch

My saga continues. i have an Intel NUC, BXNUC10I7FNK1, connected through a KVM switch to an LG 24" 4k monitor. The other computer on the switch is a first-gen ASUS chromebox, reflashed to run Linux. The ASUS box runs fine, without issues. It's on Debian Buster. THe NUC is running Debian Sid, because it needs the newer kernel, and shiny is fun. :) During boot, the monitor shows "Signal not detected" and eventually goes to power-save mode. I can watch the disk activity light on the NUC, and enter my LVM password, then my username and password, all in the blind, and get a normal desktop. If I connect the NUC to the monitor directly, I get the normal boot display, albeit at 3480x2160x60. The monitor requires a 30Hz refresh rate, so the display is flaky and drops out now and then, but it's usable just for booting. Once the Xfce desktop comes up, all is well in either configuration. I've poked around in grub, BIOS, and kernel command line options, without success. My search foo appears to be weak on this subject, because I've been searching for several days. Does anyone have a pointer to where to look for fixes? The Intel forum has been of zero help. Likewise, the KVM switch manufacturer. I'm getting a new switch sent, but it will take awhile, and I'm frankly skeptical about the switch I have being faulty.