Hardware cursor not clearing on login when using Plasma X11

Note: I used a large language model to guide me through the process of fixing this issue for myself, as well as help curating the information for this bug report. I know they're controversial, and I don't want to mislead anyone.

OS: CachyOS
Desktop: Plasma X11 (manually installed `plasma-x11-session 6.5.5-1.1`)
Display Manager: plasmalogin (`plasma-login-manager 6.5.91-1`)
Kernel: 6.18.9-2-cachyos
Mesa: 25.3.5-2
GPU: Radeon RX 7900 XTX (Navi 31), using amdgpu kernel driver
Multi-monitor: yes, 3. All the same make/model. All plugged in to the GPU (as opposed to mobo's onboard graphics).

Description:
After logging in to a Plasma X11 session, a hardware cursor image from the plasmalogin greeter remains rendered on screen at the last cursor position before login. It appears on the monitor where the cursor was when I pressed enter to log in. It persists on top of all windows and is not captured by screenshots, using `spectacle -b -r -o /.../`. Problem did not occur under Wayland.

Attempted fixes that had no effect:

- xsetroot -cursor_name left_ptr
- kwin_x11 --replace
- XFixes cursor hide/show via libXfixes
- Matching cursor theme and size between greeter and session
- KWIN_FORCE_SW_CURSOR=1 with kwin_x11 --replace (without reboot)
- xdotool mouse move

Reproduction steps:
1. Install Plasma X11 session package
2. Select Plasma X11 in session selector
3. Log in via plasmalogin
4. Ghost cursor from greeter remains at the pre-login cursor position

Workaround:
Add `KWIN_FORCE_SW_CURSOR=1` to `/etc/environment` and reboot the system