SUMMARY
After updating Arch Linux systemd package from 253.6-2 to 254rc2-2 (or later), my USB mouse and keyboard do not work in a Plasma wayland session unless I unplug and replug them. Occasionally they do work and rarely only one of the two works. This happens regardless if they are attached to the monitor's USB hub (connected via a USB/Thunderbolt3 cable), or in the motherboard's back ports or front panel ports.

The Plasma wayland session is started from LighDM using web-greeter with a multi seat setup (plasma runs on seat0).

From what I can tell the issue does not occur with the Xorg session, when running startplasma-wayland from the tty, or when using SDDM. Since I need multi-seat with auto-login on a specific seat (not the one running plasma), replacing LightDM with SDDM is not an option.

The systemd people asked me to open an issue here. The linked bug has logs.


STEPS TO REPRODUCE
1. Install systemd 254rc3-1,
2. reboot,
3. start a plasma wayland session from LightDM+web-greeter.


OBSERVED RESULT
USB input devices do not work. The mouse cursor is visible but it does not move. When the USB device is unplugged and plugged back in, it starts to work.


EXPECTED RESULT
USB input devices are working after login.


SOFTWARE/OS VERSIONS
Operating System: Arch Linux 
KDE Plasma Version: 5.27.6
KDE Frameworks Version: 5.108.0
Qt Version: 5.15.10
Kernel Version: 6.4.7-arch1-1 (64-bit)
Graphics Platform: Wayland
Arch Linux packages:
systemd 254rc3-1
systemd-libs 254rc3-1
systemd-resolvconf 254rc3-1
systemd-sysvcompat 254rc3-1
lib32-systemd 254rc3-1
lightdm 1:1.32.0-4
web-greeter 3.5.2-2 (from AUR)
plasma-wayland-session 5.27.6-2


ADDITIONAL INFORMATION
Arch Linux bug: https://bugs.archlinux.org/task/79144
systemd bug: https://github.com/systemd/systemd/issues/28512