SUMMARY
If I configure multiple keyboard layouts and set the switching policy to be per application, once the screen locks, the password prompt will use the layout of the window that was last active instead of the first (main) layout, and will not remember the last layout used for the screen lock prompt if triggered again either.

STEPS TO REPRODUCE
1. Go to system settings -> Input devices -> Keyboard.
2. Tick "Configure layouts" and add some additional layout to whatever is the current one.
3. Where it says "switching policy", select "Application".
4. Launch some random app that would allow typing on it.
5. Switch the layout to the non-default one in that app.
6. Lock the screen using windows+L, or krunner, or similar.
7. After the  screen is locked, start typing to input your password.

Optionally:
8. Change the layout inside the lock screen password prompt.
9. Enter your password so that the screen unlocks.
10. Lock the screen again, without switching to another app so that the same app would be the last active one as in the first time the screen was locked.

OBSERVED RESULT
The password prompt has the layout of the app that was last active. This will also be the case if I change the layout inside the password prompt and then lock the screen again (i.e. doesn't remember the layout for the password prompt).

EXPECTED RESULT
The password prompt should use the default layout, regardless of which app was active at the moment the screen was locked. And if it doesn't, and my switching policy is per application, then it should remember the layout that was last used for the lockscreen password prompt.

SOFTWARE/OS VERSIONS
Operating System: Debian GNU/Linux 12
KDE Plasma Version: 5.27.5
KDE Frameworks Version: 5.103.0
Qt Version: 5.15.8
Kernel Version: 6.1.0-13-amd64 (64-bit)
Graphics Platform: X11
Processors: 20 × 12th Gen Intel® Core™ i7-12700H
Memory: 62.5 GiB of RAM
Graphics Processor: Mesa Intel® Graphics
Manufacturer: SLIMBOOK
Product Name: Executive

ADDITIONAL INFORMATION