Environment

- openSUSE Tumbleweed (fully updated)
- sddm-qt6 0.21.0
- plasma6-workspace 6.6.5
- Qt 6.11.1
- SDDM running on X11
- Plasma session running on Wayland
- 3 monitors connected

Important note:
The problem has only been tested with SDDM running on X11. Native SDDM Wayland mode was not tested because it is still considered experimental on openSUSE Tumbleweed. Therefore it is currently unknown whether the issue also occurs when SDDM itself runs under Wayland.

Description
With three monitors active during SDDM startup, authentication behaves incorrectly. The user list displays both users correctly, but only one user can authenticate successfully. Selecting the other user from the user list results in an authentication failure and SDDM reports a wrong password although the password is correct.
Using "Other User" and manually entering the username allows a successful login.
After logout, that manually entered user becomes the only user that can successfully authenticate from the user list.
The behaviour effectively alternates between users depending on who last logged in via "Other User".

Journal output contains:
qrc:/qt/qml/org/kde/breeze/components/UserList.qml:32:
TypeError: Cannot read property 'userName' of null

Workarounds
1. Disable the user list completely.
2. Disable secondary monitors in the SDDM X11 configuration so that only one monitor is active during login.
After login, Plasma Wayland starts normally and all three monitors work correctly.

Steps to reproduce
1. Use Breeze or breeze-openSUSE SDDM theme or any other theme with user-preselection.
2. Connect three monitors.
3. Boot into SDDM.
4. Select a user from the user list.
5. Enter the correct password.
6. Authentication may fail.
7. Use "Other User" and manually enter the username.
8. Authentication succeeds.
9. Logout and test the other user again.

Expected result
All users displayed in the user list authenticate correctly.

Actual result
Authentication depends on the last manually entered username and the user list appears to become internally inconsistent.

Additional information
The issue disappears immediately when only one monitor is active in SDDM.
The issue does not affect the Plasma Wayland session itself after login.
Already mentioned behaviour in KDE Bug 494804.