I have a HDMI KVM switch that's supposed to be 4k@60. One PC won't detect it as such; I can make a custom 1440p at 42Hz mode but nothing higher. The other will send 4k/@60, but the monitor will do one of 4 or so failure modes:
* Entire screen compressed to half width: 50% * Correct behaviour: 20% * Correct display, occasionally flipping to black screen: 10% * Gatbled screen: 5% * No synch: 15%
Switching input is a game of toggling back and forth until it synchs correctly.
None of this happens with a direct connection.
The switch also doesn't seem to detect when I set up a keyboard macro for the switch-input sequence (control-control-1/2/3/4) but that's not nearly the dealbreaker.
I'm not sure if it's the switch or the monitor being relatively intolerant of the switch's behaviour, but I don't want to toss around $100-300 on spares to diagnose this.