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The keyboard in question is a Matias tenkeyless backlit usb keyboard ( https://matias.store/collections/slim-aluminum-keyboards-1/wired+tenkeyless ).
Importantly, this is under the Iogear GCS1204G kvm switch (product manual : https://iogear.custhelp.com/ci/fattach/get/90357/1532350710/filename/GCS1204G_Manual.pdf ). Also importantly, this keyboard works fine through the kvm switch to an 2016-era PC laptop running Ubuntu.
This keyboard works fine when directly connected to the Mac, but not on the Mac through the kvm switch.
The problem occurs on a 2013 Macbook Pro, running Mojave, all up-to-date. The external usb keyboard, when running under the kvm switch, works on the macOS login screen (meaning for example, I can type my password) but not after the user is logged in (keys can be pressed but no letters show up, for example). But the Matias keyboard is shown, when under the kvm switch, in the dropdown menu on one of the dialogs on the System Preferences keyboard panel.
Oddly, when the keyboard is connected through the mouse port on the kvm switch, it works fine on the Mac (but then of course, some mice do not work, such as the ergoomic Evoluent C that I use; a standard Mac mouse does work that way).
Regular usb keyboards such as the Logitech MK200 Media work just fine through the kvm switch.
To simplify, I have turned off Keyboard Emulation on the Mac's kvm port per the kvm switch manual (hsm mode, then press the F10 key). That did not make any difference.
Also to eliminate any possible problems, I reset the SMC per https://support.apple.com/en-us/HT201295 . That did not change anything.
Perhaps there are some command-line tools that could debug the scenario.