Unable to remap `btn_middle` to libinput using hwdb

I am using a Logitech Trackman Marble Trackball on Ubuntu 22.04, Gnome Shell 42, Wayland.

I wish to simulate a middle mouse click with one of the extra side buttons, but it seems I am able to synthesize any type of button except the middle button.

• I have created file at /etc/udev/hwdb.d/71-logitech-trackman.hwdb like so:

evdev:name:Logitech USB Trackball:* KEYBOARD_KEY_90004=btn_middle

I then run:

sudo systemd-hwdb updatesudo udevadm trigger

And physically reconnect the device.

• Running sudo udevadm info /dev/input/event8 | grep btn_middle shows that the mapping has taken effect.
• Running sudo evtest /dev/input/event8 outputs the correct mapping like so:

Event: time 1689266776.764313, type 4 (EV_MSC), code 4 (MSC_SCAN), value 90004Event: time 1689266776.764313, type 1 (EV_KEY), code 274 (BTN_MIDDLE), value 1

• However, running sudo libinput debug-events --device /dev/input/event8 does not emit any output when I press the remapped button (though it does output correct lines for all other events).

Oddly, libinput does emit output if I choose any other target event, such as btn_left, btn_right, or even other buttons that the device does not have, such as btn_back, and these clicks are correctly reflected in the GUI. It appears to be only the middle button that cannot be emulated.

The behaviour is the same if I attempt to remap any of the four physical buttons.

What could be preventing the middle mouse button mapping from working?