SUMMARY
The problem occurs when using a handheld Steam Deck in desktop mode. The lock screen provided through desktop mode's System Settings hides the Steam virtual keyboard while the system is locked, preventing the user from viewing or touching the keyboard.

This makes unlocking the system in desktop mode needlessly difficult. The keyboard can still be navigated with the controller buttons and trackpads only. With enough effort, it can technically still be navigated to input a password through audio/tactile feedback and memorizing the layout and cursor position, but this isn't practical or sufferable for everyday use.
Steps for reproducing this issue:

1.    Switch to Desktop Mode on a handheld Steam Deck
2.    Open System Settings
3.    Navigate to Personalization > Users
4.    Set the current account's password to something, such as "11111"
5.    Navigate to Workspace > Workspace Behavior > Screen Locking
6.    Check "After waking from sleep" under "Lock screen automatically:" and apply changes.
7.    Press the power button once, let the screen turn off, then press it again to show the lock screen.
8.    Press "Steam"+"X" to activate the virtual keyboard.

OBSERVED RESULT

Steam Virtual keyboard passes controller inputs to the lock screen, but isn't visible or touchable on the screen.

EXPECTED RESULT

Steam Virtual keyboard should be visible on screen and respond to touch inputs when entering the lock screen password.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: KDE Plasma
KDE Plasma Version:  5.23.5
KDE Frameworks Version: 5.90.0
Qt Version: 5.15.2