USB ports aren't getting enough power. USB mice aren't power hungry enough for that, but some USB keyboards are hungry. Unfortunately, this includes clickable M models; Results range from a keyboard that won't initialize at all, to looping when trying to initialize and failing halfway, to borderline running and crashing when actively moving the mouse. Plugging a powered USB hub between the USB port on the KVM and the keyboard gets the damn thing working, but it would be much more convenient if the KVM itself had an additional external power supply.
NEWCARE HDMI KVM Switch: 4K@60Hz, 2x1 HDMI2.0 Ports + 3X USB KVM Ports - Share 2 Computers on UHD Monitor, with Wireless Keyboard and Mouse Support - USB Disk, Printer, USB Camera Compatible
10 Review
Enhance Productivity with CKL HDMI KVM 2 Port 4K 30Hz Dual Monitor Extended Display (CKL-922HUA)
10 Review
CKL 4 Port KVM Switch Dual Monitor DisplayPort 4K 60Hz: Unprecedented Multi-Computer Control with Audio, USB 2.0 HUBS, and DP 1.2
11 Review
CKL Quad Monitor HDMI KVM Switch 4 Port 4K@30Hz with Cables - Extended and Mirrored Display Support for Windows, Linux, and Mac
11 Review