U3223QE, pin USB input to one computer for network KVM

I'm using the Network KVM function with PBP, and I'm finding that it is behaving unpredictably when I mouse over the split-screen boundary. I want to disable automatic switching of the USB-A input assignment, and only switch it manually.

Setup: I have a Macbook Pro connected to the 90W USB-C port for video and data. I have a Lenovo ThinkPad connected to the HDMI port for video and to the other USB-C port for data. Then I have a Dygma Defy keyboard and Magic Trackpad connected to one of the USB-A ports, via a hub (noteworthy, because both of these behave differently whether directly connected to a machine or proxied).

If I'm inferring correctly what it's doing:

- The monitor is presenting the devices on the USB-A port directly to one of the machines. This machine gets native access to the devices on that port.

- DDM / DDPM on each machine is capturing key and mouse events from all input devices (including the monitor's USB-A input, if assigned to the current machine).

- DDM / DDPM on the machine with focus (the mouse is on its side of the PBP boundary) is sending key and mouse events to the OS. The machine without focus receives system events (like WinKey+L to lock) but its applications do not receive input events. If the machine with focus does not have the USB-A port assigned to it, the devices look like generic keyboard and mouse.

It seems like when I cross over the screen boundary, the monitor is toggling which of the machines has the primary connection to the devices on the USB-A port. This is creating unpredictable situations for me, because the devices are interpreted differently depending on whether they are directly connected to the machine or they are proxied through DDM/DDPM. In particular, I'm seeing differences in gestures and mouse speed settings.

It seems like mousing over the PBP boundary actually causes the assignment of the USB-A devices to toggle, not necessarily to switch to the device that has the cursor. But the toggle does not happen every time I mouse over, especially if I quickly cross back.

Desired behavior: I want to default the USB-A connection to my machine on the 90W USB-C unless I explicitly switch it over with the OSD controls, or that machine is not connected. I do not want the USB-A connection to switch based on moving the mouse to the other machine's half of the PBP.

UPDATE: I created a video of what's going on here.