SUMMARY
The virtual keyboard provided by the login screen cannot be used to enter the user password, despite the keyboard visually appearing to enter keys (key pressed animation). 

Importantly, this only happens when no session exists. So it's necessary to completely log out (or restart) the system. Simply locking the current user has a different virtual keyboard behavior.

STEPS TO REPRODUCE
1. Log out or restart (Important: just locking has a different behavior)
2. Use the mouse to select the password field.
3. Click the "Virtual keyboard" button (bottom left) to open the virtual keyboard.
4. Use the mouse to click any key to enter any letter.

OBSERVED RESULT
The key in the virtual keyboard is pressed, but the letter is not input into the password field.

EXPECTED RESULT
The letter is entered into the password field, making it possible to login without a mouse.

SOFTWARE/OS VERSIONS
Operating System: Kubuntu 25.04
KDE Plasma Version: 6.3.4
KDE Frameworks Version: 6.12.0
Qt Version: 6.8.3

ADDITIONAL INFORMATION
I do not have a virtual keyboard configured for my user account. Not sure if this is relevant, since it shouldn't have access to my user setting before it actually logs me in.

I use the Breeze login screen (SDDM).

(I found this while rebuilding my PC. I forgot to plug in my keyboard and wanted to login with the virtual keyboard to quickly do something that didn't require keyboard. No such luck.)