U3219Q, KVM stopped working

I have a U3219Q that I have connected to a custom desktop PC and a Dell Latitude Laptop. I purchased the monitor in July 2020 and up until now it has mostly been a flawless experience.

A few times in the past, and now currently, the KVM feature of the U3219Q does not pass keyboard or mouse to the laptop. I still get video from the laptop. Keys and mouse and video all work fine when I switch to the desktop. In the past, some combination of unplugging the laptop, powering down, restarting, etc. would cause the feature to begin working again. I never really paid attention to what fixed it because it's happened maybe 3 times in the past 5 months and not at all recently and I can't even say for sure that the same thing fixed it every time. But I could get it working again within a few minutes.

But at this point, nothing I do will cause it to begin working again, including
- power down monitor, unplug everything including power, hold power button for 8 seconds, reconnect, repower
- factory reset of monitor
- unplug/replug USB-C from laptop
- unplug/replug Display Port from laptop
- restart laptop
- power down everything and bring only laptop and monitor up

* PC model
Dell Latitude E7470 laptop (problem) and custom-built desktop PC (not a problem)

* Operating system
Windows 10, 1809 (laptop)

* Video card specifications (video out ports)
Windows says the laptop has Intel HD Graphics 520. I have the laptop on a docking station and the video is going to the monitor via the DisplayPort on the docking station. When the monitor is displaying the DP, I have "USB Selection" setting as USB Type-C. And there is in fact a USB Type-C to USB cable running from the monitor to the docking station.

The desktop, which is working in all cases, is connected on HDMI and USB. FWIW, it has a Radeon RX 5700 XT card.

Like I say, this had been working 99% of the time. It was working earlier this morning, but when I switched back to the laptop from the desktop, the mouse and keys were no longer working. I can switch back to the desktop (using the source select button on the monitor) and the mouse and keys work fine.

This was the primary reason that I settled on this particular model and now it isn't working.

Help!