Continuing my search for solutions, I think I discovered the culprit.
The “Allow using as pointer and keyboard” option in the system’s game controller options was enabled, and one of the joystick’s miscalibrated axes was persistently throwing the mouse pointer to the right edge of the screen. Disabling this option (which I don’t know if it was active before the update) made the mouse pointer behave correctly again.
Why the pointer worked correctly with the system settings open, even with the incorrect setting, I don’t know.
The problem still persists in plasma-login-manager. I tried using the “Apply Plasma Settings” option in the plasma-login-manager configuration module, but it doesn’t seem to work.
Does anyone know how to configure this game controller option at the system level?